If you use OS X 10.5 or 10.6's built-in screen-sharing feature to work with other Macs on your network, you can control (somewhat) the quality of the image from the remote Mac by using the View > Adaptive Quality (or Full Quality) menu item. Adaptive quality adjusts the image based on the speed of the network connection, and should be the preferred choice in most situations.
But what if you want more direct control over the image quality? In early versions of 10.5, you could modify the screen sharing application to add an actual quality slider to the program. That trick no longer works, but Terminal can still give you more control over screen sharing quality, albeit with less flexibility than before.
Close any screen sharing sessions you may have open, then launch Terminal (in Applications > Utilities) and enter this command, but do not press Return yet.
defaults write com.apple.ScreenSharing controlObserveQuality
After you've pasted the above, press the Space Bar (to add a space after controlObserveQuality), then type a number from 1 to 5, according to which quality level you'd like:
• 1 = Black and white
• 2 = Grayscale
• 3 = 8-bit color
• 4 = 16-bit color
• 5 = Full color
When you launch your next screen sharing session, it will be at the quality level you specified in the above command. Resetting the quality level is quite simple -- just select View > Adaptive Quality or View > Full Quality, and you'll override the value you set via Terminal. This isn't a perfect solution, but it does provide additional control over image quality in screen sharing.