Ideas and Challenges have images associated with them that you can choose from a default list. You can add or remove from this list through the SharePoint site that is hosting the application.
This process is easily accomplished through SharePoint lists.
Here's a step by step guide:
1. Open the SharePoint site that the app is installed on (should say "Go back to..." at the top of your screen when you're in the application).
2. Click Site Contents (either in the left bar, or hit the gear icon and choose it from there)
3. From site contents, there's a list called Site Collection Images
4. From here you can see the Campaign, Category, and Gallery Folders
5. In order to delete photos, open the folder you'd like to edit and select the photos, then hit delete in the toolbar.
6. To upload new photos, hit the upload button and select the photos you'd like to upload.