Hi Sam -
Please note that the higher you compress the JPG file, the more the
quality of the image will degrade. You'll notice "blocky" areas or
blotches.
Depending on what the image is of, you may wish to change graphic
formats. For instance, if you have a large gradient, you may be better
off with a GIF file with 128 or 256 color depth and a dither.
If you can get your hands on them, Adobe Photoshop or Adobe ImageReady
are great tools for looking at the differences in compression and
format side by side. Both products will also give you a size
comparison, as well as estimate how long the image will take to
download over a given connection speed.
Cheers,
-Mark