PNG is a lossless image file type, meaning there is no compression or loss of quality when you save it. PNG files can also have transparency, or see-through areas. JPEG is a lossy image file type, meaning that even when you set the quality to 100% there is still a tiny bit of compression and loss of quality, even if unnoticeable. JPEG files cannot have transparency. For most products we recommend uploading JPEG files because they’re much smaller, and there is no visible difference in quality. However, it’s important that you keep a “working” version of your artwork such as a PSD or AI file so that you aren’t opening and saving the same JPEG file over-and-over again.