It’s very important to calibrate your monitor when viewing proofs online, or viewing digital images from your computer screen. If the color or contrast isn’t adjusted properly, the printed images will look a lot different.
Here is an 11 color scale for adjusting contrast.
If you’re really up for calibrating the gamma of your monitor, and you should if you’re going to be proofing or editing photos, you might want to take a look at a brief explanation of how to do it on zonezero. You can find his explanation here: http://www.zonezero.com/calibration/english.html.