gnuThumbnailer - more than a thumbnail generator
gnuThumbnailer is a batch processing tool for your pictures. Its primary task is to create thumbnails which can be used to build a website gallery.
The application can also be used to edit multiple photos with a few clicks.Therefore, gnuThumbnailer has some image editing features. For example, the application can be used to color pictures (black and white, sepia, user-defined color overlay), to flip and rotate them (also automatically by EXIF information) or to add a water sign or a border.
Here you will find more information about gnuThumbnailer.
You should also check out the screenshots.