Your computer probably has an image manipulation program--it may have come with your camera.

There are also some online services. I haven't used them so I don't know anything about them, but they're free:

http://www.jpegwizard.com/

http://www.onlinephotographers.com/shrink/

http://www.netmechanic.com/GIFBot/optimize-graphic.htm

...and what looks like a pretty good one:

http://www.shrinkpictures.com/

Lynn Siprelle, Editor