Preview Credits are used automatically each time you create a new preview while you're signed in and have a credit balance of Preview Credits. Each time you generate another preview (including trying the same photo in a different style or palette), one Preview Credit is used.
If you're not signed in, your first preview in that browser may be offered without using a Preview Credit (one-time per browser); after that, sign in and purchase Preview Credits to continue creating previews.
Digital Download Credits are used when you're ready to unlock the full-resolution version of a preview. Find the preview in My Art Gallery and click Use Credit to instantly redeem one and download the full-resolution file.