Walter can't take a screenshot anymore using Assistive Touch in iOS 8. Something had been changed from iOS 7. He wants to take a screenshot of photos that he has zoomed in on to get rid of the black borders around them.
Since the camera roll in iOS is accessible from other apps, Leo suggests trying SnapSeed. It's a free Google product, and it's a photo editor that has cropping. This is a great photo editor that does a lot of things, so it's worth a try.