Amazon's Merch on Demand upload guidance is short and specific: an image in JPEG or PNG format, at least 1000 x 1000 pixels, less than 25 MB. Notice that the pixel rule is a floor, not a target — the guidance says higher quality is better and that photos taken on phones work great while images downloaded from the internet typically do not. So the useful compression here is narrow. You only compress when a design export blew past 25 MB, which mostly happens with textured or photographic artwork saved as PNG, or when you are uploading a batch and want consistent, fast uploads over a phone connection. What you must not do is resize below 1000 px to hit a byte target, because that trades the one hard requirement for the soft one. Skwish lets you compress to a MB target and resize to exact dimensions separately, so you can pull a 30 MB PNG down to a few MB while the shortest side stays comfortably above 1000 px. Everything runs in the browser; nothing is uploaded to a server. Also worth keeping in mind: Amazon's content guidelines are about what the design depicts, not its file size, and no compressor will save a design that infringes a trademark.