How to use AI image generators, professionally post-process your artwork, and prepare it print-ready for Print-on-Demand
Imagine you have an idea for a T-shirt design — an enchanted forest in neon tones, an abstract lettering piece, or a retro comic-style illustration. Not long ago, you would have either spent weeks drawing it yourself or hired an expensive illustrator. Today, a few precise text prompts and the right AI tools are all it takes to have a print-ready artwork in your hands within hours.
AI image generators have fundamentally changed the way creatives work. What many people still don't realize: the generated images are just the raw material. The real value is created in post-processing — and that's exactly where your existing knowledge of Photoshop and Illustrator comes into play. Mastering this workflow gives you not only a creative edge, but also a real monetization channel: with a platform like Print-on-Demand by Spreadshop, you can start selling your finished designs on apparel, mugs, and more right away — no inventory, no minimum order, no upfront costs.
In this tutorial, I'll walk you through the complete workflow: from prompt strategy to Photoshop post-processing to the final print-ready export.
What You'll Need
Before we dive in, here's a quick overview of the tools used in this tutorial:
- Midjourney (via Discord, from approx. $10/month) or Adobe Firefly (included in Creative Cloud)
- Adobe Photoshop for background removal, resolution upscaling, and finalization
- Adobe Illustrator optionally, if you need your artwork as a vector graphic
- A free Spreadshop account for uploading and selling
Step 1: Writing Prompts That Work for Textile Printing
The biggest mistake when using AI image generators for merch is simply asking for "a cool T-shirt design." The result is usually too complex, too detailed, or has a background that's nearly impossible to remove cleanly.
Three principles for merch-ready prompts:
1. Clear background from the start Always explicitly state: "on a plain white background" or "isolated on white." This saves you significant time when removing the background in Photoshop later.
Example: vintage eagle illustration, bold linework, limited color palette, retro americana style, on white background, t-shirt graphic design
2. Style before subject Name the visual style first, then the motif. AI generators weight earlier parts of the prompt more heavily.
Weak: a wolf howling at the moon in a cool style Better: woodcut print style, dramatic contrast, a wolf howling at a full moon, bold black outlines, limited 2-color palette
3. Think about resolution and format In Midjourney, use the --ar 1:1 parameter for square designs or --ar 3:4 for portrait-oriented artwork. Also add high detail, print-ready to your prompt — this measurably improves output quality.
Ready-to-use prompt template:
[Style], [mood/atmosphere], [main subject], bold graphic design,
limited color palette, clean edges, isolated on white background,
t-shirt print, high contrast --ar 1:1 --v 6
Step 2: Making Your AI Image Print-Ready in Photoshop
Even the best AI-generated artwork rarely comes out of the generator print-ready. Three tasks almost always await you:
2a. Bring the Resolution up to Print Standard
Midjourney and Firefly typically output images at 72–150 dpi — not enough for textile printing. Spreadshop recommends a minimum width of 4,000 pixels for front-of-garment placements on standard apparel like shirts and hoodies, and at least 2,500 pixels for smaller products like mugs or stickers.
How to do it in Photoshop:
- Open your AI image in Photoshop
- Go to Image → Image Size
- Enable Resample and choose Preserve Details (Enlargement) as the method
- Set the width to at least 4,000 px at a resolution of 300 dpi
- Check the result at 100% view for any loss of sharpness
Alternatively, the Firefly Upscale feature (directly in Creative Cloud) or a dedicated tool like Topaz Gigapixel AI delivers even better results, especially for portraits or fine textures.
2b. Precise Background Removal
Even with an "isolated on white" prompt, a slightly gray or uneven background often remains. Photoshop offers two quick methods:
- Select Subject (under Select → Subject) works very well for clear silhouettes
- For more complex artwork, the combination of Select → Select and Mask with the brush tool is recommended to cleanly separate hair or soft edges
Save the result as a PNG file with a transparent background — this is the preferred file format for uploading to Spreadshop.
2c. Check Your Color Mode: RGB vs. CMYK
AI generators always work in RGB. Many print platforms — including Spreadshop — accept RGB files and convert internally. Still, it's worth a quick check: very vivid, saturated colors (especially neon tones) can appear more muted after printing. When in doubt, reduce saturation slightly or use Photoshop's Softproof view (View → Proof Colors) to preview the result.
Step 3: Optional — Vectorization in Illustrator
If your artwork consists of clear shapes and lines (e.g., an icon, a lettering piece, or a simple character design), the step into Illustrator is worth taking. Vector graphics can be scaled without any loss of quality — perfect for screen printing, or if you plan to use the same design across other formats (posters, stickers, embroidery patterns) later.
Workflow in Illustrator:
- Place your isolated PNG via File → Place
- Click Image Trace in the top toolbar → [Preset: Black and White Logo or 6 Colors]
- Click Expand to make the vector editable
- Clean up unwanted small paths via Select → Same Fill Color and delete the fragments
Export the result as EPS or a high-resolution PNG (with transparency enabled).
Step 4: Upload and Product Configuration on Spreadshop
The upload process on Spreadshop is designed to be intuitive:
- Log into your Spreadshop account and select Add Design
- Upload your PNG (min. 4,000 px, transparent background)
- Place the design on your chosen product: the live editor instantly shows you what the result looks like and lets you position the artwork within different print areas. On T-shirts, for example, you can typically place designs on the front, back, and often even on the sleeves.
- Choose product colors that showcase your design to its best advantage (dark designs on light shirts, light designs on dark ones — sounds obvious, but it makes an enormous difference)
- Set your selling price: Spreadshop deducts the base cost, the rest is your commission
Clear product titles and descriptions also help make your shop easier to navigate, especially once you start sharing it across social media, concerts, or your website.
Common Mistakes — and How to Avoid Them
| Mistake | Why it happens | Solution |
| Design too detailed | AI tends to generate complex backgrounds | Add "bold, simple, limited detail" to your prompt |
| Resolution too low | Standard output is usually only 72 dpi | Upscale to 300 dpi / 4,000 px before exporting |
| Trademark infringement | AI mimics known styles and logos | Never prompt for real brands, logos, or characters |
| Unclean background | Slight gray tones from AI artifacts | Always check background removal in Photoshop, save PNG with alpha channel |
| Colors look muted after printing | RGB → print color space difference | Slightly reduce saturation, use Softproof |
Legal Note: What to Keep in Mind with AI Designs
AI image generators are not a legal gray area. Under the principles of the German Copyright Act (UrhG), the following applies in Germany: anyone who creatively shapes a work acquires protection over it — the more of your own creative contribution you bring, the stronger your legal standing. Key points for merch designers:
- Never prompt for well-known characters or brands — even if the AI generates it, copyright or trademark infringement can still arise
- Midjourney commercial rights: with a paid plan, you receive commercial usage rights to your generated images — with the free plan, you do not
- Adobe Firefly is specifically trained and licensed for commercial use — according to a recent comparison by t3n, a clear advantage over Midjourney when it comes to legal security
- Original, unique prompts combined with substantial post-processing strengthen your creative contribution to the final work
When in doubt: the more creative effort you put in (post-processing, composition, color work), the clearer your authorship of the final result.
Conclusion: AI as a Creative Tool — You as the Designer
AI image generators are not a threat to designers — they are arguably the most powerful prototyping tool ever developed. Those who deliberately refine the output with Photoshop and Illustrator produce results that no pure AI output could ever match — while simultaneously building the foundation for a scalable product portfolio.
And that's exactly where Print-on-Demand becomes so powerful: instead of letting those finished designs sit unused on your hard drive, you can immediately turn them into real products people can buy. No inventory, no minimum order, no risk. You design, you upload, you sell — the rest runs automatically.
The best time to start was yesterday. The second best time is now.