Sometimes, images, music, and other external content can contain "beacons", which are used by spammers to verify your email address so that they may send you more (and more) spam. With its 2007 version, Outlook Web Access attempts to prevent this by blocking anything that could contain a beacon. You will probably see a warning with a Click here to unblock content option. If you trust the sender, go ahead and unblock the content. If you don't, the images will be blocked when you view the email and the beacon (if one is there) will not be activated.
It may be a bit of an inconvenience, but the blocker helps to protect the computer and, most importantly, your privacy from people who want to inconvenience you even more. Though it is not recommended, you may turn off the blocker ( Options>Privacy and Junk E-mail Prevention ).