PNG comes in two kinds. A full-colour PNG stores every colour the photo holds; a palette PNG keeps a short list of colours to make the file lighter. Some export presets and online compressors switch to the palette kind without saying so. On a product photo this shows as stepped bands in shadows, gradients and folds of fabric, so the full-colour kind is the one to keep for photographs.
For a photo of a product on a solid background, PNG is often several times heavier than a JPEG that looks the same, and the extra weight can push the file over an upload limit. Where PNG earns its place is in images with text and hard edges, such as size charts and care-label graphics, and in cut-outs that have to stay transparent. Shop systems differ in which of the two formats they recommend for product photos.
A PNG is lossless only from the moment it is saved. A photo that was a compressed JPEG before keeps its blocks and blurred edges when it is saved again as PNG; the file just gets heavier. Converting to PNG therefore never repairs an image, and the source for a PNG should be the original photo or a lossless working file.