JPEG.

A lossy raster format with no alpha channel, so every pixel is solid. On a garment that is the whole problem: Printify says JPEG forces a solid background, usually white, which prints as a white box round the design on a dark shirt, and its DTF checklist reads "No JPEGs, ever". Transfer Superstars accepts JPG as the last resort and warns of a white background baked into the transfer. On wall art the same property does no harm, because the print covers the whole sheet, and Printful names JPEG for full-coverage photos, posters and canvas. Every re-save recompresses the image, so a JPEG passed between designer, seller and shop arrives with less detail than it left with.