When uploading your Templates into your platform you have the ability to decide if you want to your end users to be able to upload replacement images for the templates from their desktop, or restrict them to only being able to do so from images in their Resources within your IntelligenceBank platform. You can even take that a step further and only allow images that are Related to the template.
When clicking to either Add New Resource (and then selecting the InDesign Template option) or to Edit your existing Web-to-Print Template you will see the below options towards the bottom of the page:
To enable users to replace Template images from their Desktop, select the Desktop option.
To restrict users to only being able to replace Template images from one of these sources, select the applicable option/s.
If there are no restrictions on where users can upload images from, you can leave as all.
Remember to Save your changes.