Default wallpaper appears after startup or restart in Yosemite 10.10.3
After installing Yosemite 10.10.3, when I start up the computer or restart, my usual background image is replaced by the default Half Dome wallpaper image. If I open preferences and go to the wallpaper section, I can see my usual image there in the wallpaper box. I can reselect it and the wallpaper will change to the image that I want, but after I restart, etc it goes back to the Half Dome image. This never happened before upgrading to 10.10.3. I tried deleting the desktop preferences file but it didn't help.
In terms of possible computer problems this one is admittedly low on the headache list but I'd still like to get it resolved. Thanks.
MacBook Pro, Mac OS X (10.6.1)