I wonder if the problem I am having is the same as yours. I upgraded a system from F7 to F8. I have a logout
button in the panel, and after the upgrade instead of the logout icon I get one that looks like a dark photograph
with the lower right corner turned up. If I go to add a logout applet to the panel the menu shows the green running
man icon, but when I add it I still get the dark photo icon instead of the running man.
Well I just looked this up on bugzilla and found bug 432245 seems to be on this topic.
If I made the top gnome panel size 47 pixels or less I get the green running man logout
icon. But if I make it bigger the icon turns into the dark photo with the lower right corner