You'll need an image editor. I use Microsoft Picture It (not popular but I got it for free). There are a number of free shareware programs out there as well. Just Google free image editor. Then you just format the picture to a lower resolution, you may also want to crop the image so it's smaller further reducing it's size.
Frankly, I cannot possibly fathom working on eBay with a 56k modem. Or doing anything else on the web for that matter. You are editing images to suit your needs. Most of your viewers will have high speed connections and see the lower quality of your images. I know a lot of people say to consider users who have dial up when preparing your auction listings but frankly I don't give it a moments thought.