Hey, I've been looking for the same thing for the past few days, and I found a great solution. Check it out;
Batch Picture Resize
It's really great. I needed all my images at 800px or less, but I didn't want to go through too much trouble. This is awesome - it doesn't take up any space AT ALL. The size of the image is controlled by the name of the file. Idk, check out the page; it'll give you better info than I can.
P.S. Rolled across this post when I was looking for something good. You're the sixth result under "bulk resizing". Just thought I'd help out