You get the best result with a file that is full size on 180 DPI. You have to save your file as a jpg and color mode: sRGB. You can upload files up to a maximum size of 150 MB.
Pay attention: the size of a file is determined by the DPI. The file always opens on 180 DPI. This means that if you use a different DPI, the size of your file will change when you upload the file in the Webshop.