If you like messing with your vanilla photos (who doesn't, right?), this app has almost everything you need. Collage? Check. Weird filters? Check. Effects so numerous you'll waste hours procrastinating? Check.
I can say that “PicsArt“ represents the most complete image editing-program par-excellence and it's available for free!
P.S.: There are also In-App purchases to finance the app!
Also optimized for tablets!