There's a tool called David's Batch Processor which allows you to batch process images within the free Graphic Image Manipulation Package (GIMP).

To install on Ubuntu Hardy, first...

sudo aptitude install libgimp2.0-dev

Now download David's Batch Processor from here

Extract the file and Terminal into the directory, now make & install the plugin...

sudo make
sudo make install

...if it still doesn't compile make sure you have build-essentials installed, which can be added via,

sudo aptitude install build-essentials

After all this open GIMP (Application / Graphics / GIMP), Click the Xtns tab (next to File) and you should see Batch Process..; click it and you'll open David's Batch Processor.