Android (e.g. Samsung, HTC, Sony): To delete all photos from a group album, long tap on a photo to mark it. When it’s marked, tap the symbol next to the trash can in the top right corner to mark all pictures. You can then delete the entire history by tapping the trash can. If you only want to delete one photo, simply mark it then tap the trash can symbol.

iOS (iPhone): Long tap on a photo, then select either delete (one photo) or delete all (entire album).