Once you prepare your images according to the specifications of the prints you want, upload them in the print files section. You can upload several files at once.
Give each file a name and associate it with one or more products. A single file can yield multiple prints. For example, a single 36x48" image can be used to create a 36x48" Art Paper print and a Luster Print without uploading twice, along with smaller sizes of the same format (say 18x24" and 9x12"). If you've connected a store, you'll be asked to associate each upload with a product on your store.
Note: keep in mind that all files must follow the basic requirements in order to work.