Sift

This app lets you swipe through the photos/videos on your phone and at the same time decide which ones you'd like to keep and which ones to delete. Swipe right to keep, or left to delete.
Whenever you feel like it, you can display all photos marked with "delete" - and confirm to delete them with one click. Off course you have the option to change your mind on one or several of the photos marked "delete" before the actual deletion.

If you like me always take more than one picture at the same object to make sure you get at least one good shot, this app helps you decide what version to keep, by easily swap currently shown picture with last one.

If your camera stores the photos on an external SD-card, you have to add the camera directory to give write permission to the app by selecting "Options/Add media-directory" and brows to your camera-folder (see pictures in app-description)