To remove your image from a Collection:
Tap on 
and then on 
Tap on the published image you would like to remove.
iOS
Swipe left on the user who republished your image and then tap on 
to remove the image from their Collection. Please note that you made need to swipe a few times for the delete option to appear.
Android
Tap on the published image inside the Notification center under the Activity section. Tap Remove to remove the photo from the user's Collection.
Once you have removed your image from another community member’s Collection, they will not be able to save or publish that image again.