Gallery is the app to display photos by scanning media files from your phone. What you need is to stop Gallery from scanning a folder (which is usually related to an app), thus hide those photos from Gallery. You should first find out the location/folder of the photos you want to exclude from Gallery, then either create a file ".nomeida" (without quotes) in the folder by using a file manager, or just add "." (without quotes) to the head of folder name.