Don’t blame me if your life becomes worse 
(Joking, it’s great for any kind of image manipulation that you do more than once.)
Recently I’ve hacked together an AppleScript that does some simple (but nice) things via ImageMagick. May serve as a showcase or as starting point:
IM-Screenshot-Prettifier.scpt.zip (21.6 KB)
(For the script to work you need to have ImageMagick in /usr/local/bin, as well as the other two dependencies mentioned at the beginning of the script.)
To make full usage of the script launch it with this KM macro: IM-Screenshot-Prettifier.kmmacros (3.4 KB)