This can occasionally happen when OSX recovers from a deep sleep, particularly on dual-monitor systems where there already issues with the cabling.
The fix:
Open up Terminal and type the following:
sudo killall Dock
That should restore the wallpaper for both terminals.
I ran into another problem with my Viewsonic secondary monitor related to recovering from sleep mode. Occasionally, the monitor wouldn’t be recognized as being online so OSX would ignore it, shoving all the content from that monitor to the primary. Unplugging and reseating the MDA from my Mac Pro didn’t fix it it but, oddly, reseating the HDMI connector at the monitor end did. I know this is a cabling issue but until I get around to investing in another HDMI-MDA cable this does the trick.