If you have already added the My Pictures folder on a single PC to the Mesh, you will see the folder on the Live Desktop and it is offered to the other PCs in your Mesh. On the Live Desktop, change the Sync options for the folder to not sync with the Live Desktop and it will only be available peer to peer between computers.
Note, however, that you really need to make sure that your Pictures are backed up somewhere other than the Mesh folders as this is a beta service and data loss, while rare, can happen!
-steve
~ Microsoft MVP Windows Live ~ Windows Live OneCare| Live Mesh|MS Security Essentials Forums Moderator ~