Layers

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So, I've decided that I really need a new software for my pixels-this is because the software I use (Paintbrush) doesn't have layers, meaning when I scan a sketch in that I did on paper, the background is not perfectly white, so I have to go over EVERY pixel, without the use of the fill can. Lots of people make their sprites/pixels using photoshop but my trial has run out :c Does anyone know of a good program that uses layers, is compatible with Mac and is free/has a trial? Thank-you for any replies!

EDIT: Thank-you very much for all your ideas, I downloaded quite a few to try out! I also found Pixelmator while searching another one, which is really good too!)