GIMP

GIMP /ɡɪmp/[4] (GNU Image Manipulation Program) is a raster graphics editor[5] used for image retouching and editing, free-form drawing, resizing, cropping, photo-montages, converting between different image formats, and more specialized tasks.

GIMP is released under LGPLv3 and GPLv3+ licenses and is available for LinuxOS X, and Windows.