The "Fast Image Resizer for Java Tool" has following features: resize all JPEG/JPG images of a directory, generate web galleries, store resized images in a ZIP file, add text or image watermark. The Tool is available in German and English.
Testlab for Image Processing: You can enter formulas like "r = r + b/2; g=g*2; b=b/2" and watch/store the Result. Alternatively cou can run a convolution matrix on the source image.
Implemented in Java, fully GUIed, avg. performance.