Before choosing a product (Uploading via "Accounts>Upload images"): 

You can upload photos from your device's photo gallery (phone or tablet), into your existing Photobox albums. To do that, click on "Accounts" (from the menu at the bottom of the page) and then "Upload images". Select the album where you want your photos to upload to. You will be able to use photos from your account on our website.

After selecting a product: 

When you have chosen your product, you will be able to select your photos. You can choose photos from your device, Photobox albums (provided you're logged in), Facebook, Instagram or Google Photos.