Electricsheep has been causing me grief in previous and current (mint 13) versions of xfce. There is one partiall (in my case) fix to have it added to xscreensaver menu--by adding,
-GL electricsheep --root \n\
to the end of the list in the .xscreensaver file. This wasnt working for me.
Just by chance I went into the preferences of electricsheep whilst in xscreensaver and changed the renedering type to "any" instead of GL,BINGO a working (full screen) electricsheep.