GIMP - GIMP is a multiplatform photo manipulation tool.
XnConvert - XnConvert is an easy image converter for graphic files, photos and images available on Windows...
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.
BatchPhoto - BatchPhoto is a batch photo editing software for Win and Mac that can convert, resize, annotate and watermark hundreds of photos in one go. Learn more and try it for Free!
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
GraphicConverter - Graphic Converter for Mac and Windows, can converts image formats BMP, JPG, GIF, ICO, PNG, TIF...