This may or might not need to be done, depending on future updates for this theme. Go to Utlities, Add/Remove Modules and see if “Product Image Custom Fields” is in the “Available Modules” area. If it is, click on the install button. If it’s down below under “Installed Modules” then you can skip to the next step.
The Prepworx Theme, in addition to the pkg file, also has an excel file that needs to be uploaded and installed. This excel file will allow you to add four custom text fields: