People often want to know what an application looks like before they install it. This is one of the reason why Christoph Haas started with the screenshots.debian.net service. People can upload screenshots for their favorite applications for others to take a look at.
Finally, as convenience feature, they are now added on packages.debian.org. When you click on the screenshot of a package you can see the list of all of the available ones.
In the case when no screenshot is available yet the page will show a placeholder image which gives you a convenience link to the screenshots page of the package where you can submit one for the benefit of all. Please make sure that you are following the guidelines for submissions, otherwise your upload might get rejected.
On a not totally unrelated topic, the packages.ubuntu.com site finally will also show you packages in maverick. It doesn't show screenshots yet—if you think adding support for screenshots in there would be a good thing too, please let me know!