The pixel dimensions probably don't matter as much as the file size. I use Photoshop which has a "Save for Web" feature. I can take an 800x640 pixel image and reduce the file size from 200kbps to 80kbps while retaining the 800x640 pixel size. The quality just goes down a bit, but not enough to matter for the web.

I'm sure there are some free jpg optimizers out there. In fact, here's one:
http://www.freedownloadscenter.com/M...Optimizer.html

Just Google for jpg optimizer