Screenshots for iPhone 4/5/6/6+/iPad
There are solutions to automating editing and generation of App Store screenshots. Whether you find them 'simpler' depends a bit on what tools you're comfortable using:

Editing existing screenshots?
If you're adding frames and title texts, you can automate this with FrameIt (if you are comfortable working in the command line).
Other generic image editing problems: Use Adobe Photoshop and make friends with the Actions palette.

Generating new screenshots?
Automate generating new screenshots across devices and languages with Snapshot, this relies on learning a bit of the Instruments app and specifically UI Automation, and some command line fiddling.

If I were to recommend, I would work through automating generating new screenshots rather than fiddle with editing existing screenshots. This might have a harder learning curve, but once done you should save a lot of time when preparing screenshots for future versions of the app.

Snapshot and FrameIt are both part of Fastlane toolset for automating the App Store submission process.