Media Center is inconsistent in how it uses the "Libraries" of Windows 7. Movies, Music, and Recorded TV work with their respective libraries the way you would expect. However, any folders added to the Pictures and Videos libraries just show up as folders under Pictures and Videos in the Media Center user interface. The frustrating thing is, this is not how it worked with MCE 2005 Public Folders. In MCE 2005, if you set Public Pictures to a network share, the Media Center interface showed you the contents of the folder in Pictures, not just a single folder with the network share name. Windows 7 has gone backwards on this functionality. I logged this "bug" early on in the Windows 7 beta and was told it was "by design."
The above solution (may) work, but it won't help you with extenders. You'll have to go digging in the registry for the extender's profile to set the My Pictures and My Videos folders. Also, if you don't have a dedicated Media Center computer, such as an HTPC, this makes working with photos and videos outside the Media Center domain more difficult.