When adding a new screen via xrandr --output VGA --right-of LVDS --auto and this new screen uses a different resolution (1680x1050 here, vs. the 1440x900 from the LVDS/laptop) the desktop background is "tiled". I see the complete background image scaled to 1440x900 in the upper right corner of the new screen and then the left part of the image on its right filling the gap between 1440 and 1680. And similarly another piece below and in the bottom right corner a third one.
Ooops, after moving the window that was partially on the new screen back to the first screen the desktop wallpaper is gone. So I just filed a dupe of my own bug (164107) *** This bug has been marked as a duplicate of bug 164107 ***