Well the topic says it all...
Nvidia GPU here, not sure if it happens with Ati or Intel.
This bug started to happen like 2 or 3 days ago either with daily build or installed/updated to HD.
Just go to: settings / appearance and change ANY item like style, icons or fonts and you lose your background/image.
The only way to revert is log out/in or go to desktop settings and uncheck/check "backgorund tab / aplly to all workspaces"
Hope it gets fixed