GIMP - GIMP is a multiplatform photo manipulation tool.
Resizing.app - Image resizing has never been so simple with free online resizing.app. Convert, optimize, resize images with ease and for absolutely free.
Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
Squash 2 for Mac - The easiest way to compress and optimize images for the web
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.