GIMP - GIMP is a multiplatform photo manipulation tool.
ImBatch - ImBatch is a batch image processor with a nice graphical user interface.
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.
DVDVideoSoft Image Convert and Resize - Free Image Convert and Resize is a compact yet powerful program for batch mode image processing.
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
Batch Image Resizer - Resize, crop, shrink, flip, exif-rotate, convert, enhance, process multiple pictures and photos with professional software! 120+ Actions, 30+ Image Formats