Different programs can produce surprisingly divergent results when exporting pictures for the web. For example, for two thumbnail images of book covers I recently posted there was a 3x size difference depending on which program I used. One of the images took 1/3 the space when compressed using Preview.app, the other when using ImageMagick. It's good that I looked at the file sizes before posting, and knew to try another program.

If the image you are compressing is a PNG, try pngcrush. Depending on the image, you can usually save about 15% of the filesize.