Already rejected
Upload rejected: resolution too low
The conclusion first: this cannot be fixed by enlarging the image. Missing pixels means the information was never captured, and upscaling algorithms invent what was never there.
First, know which number is being measured
Platforms do not read the DPI tag in your file. They divide the pixel count by the print-area inches. An 800px-wide file printed across a 15-inch area is 53 DPI whatever the tag says — and editing that tag changes nothing at all.
Batch, auto-fix and exports
Everything above is free for one file at a time, and always will be. Doing it to a whole folder — batch checking up to 100 files, one zip with every platform file for every design, and a CSV report — is part of Seller.
Exports are padded and centred to each platform's size on a transparent canvas. Nothing is ever upscaled — a file enlarged to pass a dimension check still prints soft, which is the failure this tool exists to prevent.
What each platform actually enforces
- Printful — no single size; set per product. Floor is 150 DPI, files cap at 200 MB and 20,000 px
- Printify — no platform-wide size at all; product → provider → print area. 300 DPI recommended, 100 MB for PNG/JPEG
- Redbubble — per product: Graphic T-Shirt 3873 × 4814, Premium T-Shirt 2875 × 3900. The "3000px long edge" rule is not theirs
- Merch by Amazon — the exact-size rule was dropped; anything above 485 × 485 is accepted and scaled. PNG, RGB, under 25 MB
What actually fixes it
Go back to the source and re-export. Vector sources (AI, SVG, Figma) scale to any size losslessly. If the source is raster and simply does not have the pixels, the artwork has to be redrawn or reshot. Every tool advertising "AI upscaling to print quality" falls apart on physical product — crisp on a monitor, soft guesswork on fabric.
Frequently asked questions
Does editing the DPI value in file properties help?
No. That is a metadata field and it does not change how many pixels the image contains. Platforms calculate pixels divided by print-area size.
Can an AI upscaler rescue the file?
Not for print. Upscaling generates statistically plausible detail rather than detail that was captured. It reads acceptably on screen and prints visibly soft.