desktop picture suddenly changed by itself
I woke up the morning turned on my mac and booted it to find that the desktop picture was the blue galaxy.
When I shut it down last night it was set to the purple nebula as far as I can recall. I havent enabled "change desktop picture every 30 minutes" so I'm very puzzled as to why it has randomly changed itself. The only I did yesterday was install a couple of Apps from the App store. A photo duplicate finder - and an application uninstaller...
I'm almost wondering if I have downloaded a trojan or back door open - ridiculous I know as my security is pretty paranoid and none of my safeguards has reported anything, nor do I get software from non official sources, but I'm at a loss to figure how this has suddenly changed iteslf... I mean computers don't do that... do they?
Anyone who can explain what has happened I will be very grateful.
MacBook Pro (13-inch Early 2011), OS X Mountain Lion, 4 Gb Ram 512gb HDD