Most AI shirt generators will happily show you a nice mockup and then hand you a flattened JPG. Drop that on a black tee and you get a pale rectangle floating around your artwork. I once spent an entire evening re-cutting backgrounds in Photopea because of this.

A transparent PNG means the alpha channel is intact, so the design sits on the fabric instead of on a box. It also matters for print-on-demand. Printful, Printify and Gelato all expect a PNG with transparency if the shirt color is supposed to show through. A JPG uploads fine, but you'll see the edge in the proof.

What I check before trusting any generator:

- Does the download come out as .png, or a .jpg with the extension renamed?

- Is it flat artwork with a clean edge, or a photo of a shirt pretending to be a file?

- Any watermark or "upgrade to remove" step at the end?

- Enough resolution to print — 4500px on the long side is the safe target.

ShirtCraft AI is the one I keep coming back to. You type something like "retro mountain badge, two colors" and it returns a design you can download as a transparent PNG. Basic use doesn't require an account, and there are no design skills involved, which is the whole point of these tools.

It's free at https://shirtdesigngeneratorai.com and the file drops straight into a print shop without a detour through an editor. That's a low bar, and somehow most tools still miss it.