This question may be related to a resolved thread.
Kevin mentions here in this thread http://ellislab.com/forums/viewthread/205525/#965611 that you have to “check” “Display member avatars in entries” to get {photo_url} to work.
I think this is an unexpected behavior.
I don’t see it addressed in 2.5 change log. Should I create a bug?
[edit]
Just to confirm, guests can see {photo_url} just fine but logged in users with that box unchecked cannot.