Super Mario Bros. Crossover is a Flash game where you play Super Mario Bros. levels (and more) as various classic characters (Mega Man, Simon Belmont, ...). There are also different "skins" for the characters (Mario SNES sprites, Trevor Belmont, ...), which could be submitted by users to the development team to implement or not.
I've done and submitted various pixel graphics for characters skins, collaborating with other users on sprite sheets. Those graphics were based on existing graphics and chara designs from other games, but were custom-made to fit the Super Mario Bros. Crossover skin templates. I am credited in the game and on the credits page as "Rey D" (http://supermariobroscrossover.com/resources/credits.82/).
In this porfolio, on each image, the work I've done is highlighted by white borders and lighter grey background; the left part is the design basis, and the right is the template.