DVDVideoSoft Image Convert and Resize - Free Image Convert and Resize is a compact yet powerful program for batch mode image processing.
Photoscape - high quality photo editing software, that enables you to fix and enhance photos.
Darktable - darktable is an open source photography workflow application and raw developer.
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
Caesium Image Compressor - Compress your pictures up to 90% without visible quality loss.
GIMP - GIMP is a multiplatform photo manipulation tool.