If you have either an Android device or iOS and need a really comprehensive efficient image editor; export or save the screen shots (any image) into you iPhoto/google drive or Dropbox etc and then give Google SNAPSEED - (iOS Link) a go!.
I have used Adobe CS (Photoshop) etc professionally for years to edit compile images but I find this app really useful and nifty is the best for quick on the fly adjustments for work or fun and is extremely versatile - It doesn't give an Instagram/ready-made one click template feel to your pictures (unless you choose that vibe). If you want to use built in apps then iPhoto should be really fine for basic editing.
With GIMP — unless you have used packages like Adobe CS before — you will have lots of homework to do but it's fun free and does everything that Photoshop does in pretty much the same way or simpler once you work your way around it!
Sorry I don't have before and after for all or in order