From Cerebral Synergy:
BIMP Lite is a small and simple batch processor for image files, that allows you to perform various functions on multiple images. It can be used to create thumbnail images from your pictures, add a prefix/postfix, replace spaces in the file name with underscores, flip or rotate images and also apply anti-aliasing and bevel effects. The thumbnail resizing allows you to force a specified width or height and keep the proportions, or define fixed image dimensions. In addition, the program can also convert the file format to GIF, JPG, PNG, BMP, MIFF, TIFF or TGA. All of the actions can be performed individually, or combined.