To remove photos from your frame open the Aura App and click into your "Added" photos. Here is where all the photos that have been selected for your frame live.
To remove a photo from one of your collections, tap on the collection and tap the icon on the top right (three small dots) where you will see the option to 'Hide Photos' or 'Block Album.' Selecting to 'Hide Photos' will allow you to check and remove individual photos from a collection however 'Block Album' will remove the album entirely.
To remove individually selected photos from your frame, tap the photo to view the photo details page. On the bottom right you will see the option to 'Remove' the photo from the frame.