The size of the images that you upload is entirely up to you, however, our recommendation is to upload images somewhere around 3,000px on their largest side.
This size is (a) sufficient to display nicely on all screens, (b) it's large enough to work well for the majority of all print orders, and (c) it's large enough to be offered as a high-resolution instant download.
If you receive an order that requires a larger version of a specific image (for a large print, for example), you can simply upload a higher resolution copy of that specific image when it's needed.
Note: Uploading images that are smaller than 1281px on their largest side is not recommended.