Personally I'm using PhotoFiltre, and some users tend to refer to my graphics like: wow, are they designed by photoshop? Nope just by innovative minds and pics layered with some added text, effects, etc. It's a free app for download in english (I guess originally being coded in french) language, I guess somewhere there's also a portable version with less space as file, could be used on USB-sticks & the version I tend to like being "7", which is rather outdated, anyway I like its features, while soe other/previous versions including different effects but no layer-options.
I was also suggesting this program to get included in "apps used when developing your game", because the banner / in-game images / screenshots, do somehow refer to programs, which aren't used for coding.