Did your desktop change right in front of you (one moment YOUR desktop, then some random change to something else?)
When you say that it changed - did it change to the original default background?
Another indication could be that your Dock does not look the same.
Have you noticed anything else that might mean that your system settings are reset - things like screen resolution, window settings, or even the menubar
Have you noticed other changes recently, such as much slower perfomance, or sudden freezes, when you have to force a shutdown?
Any of that can be a symptom that you may have a hard drive that is beginning to fail - or perhaps you have a permissions issue...
(Even in the unlikely event that your computer is hijacked, a malware scan with Sophos, etc, won't report anything about that)
Boot to your recovery partition, and run Disk Utility/Repair Disk.
Then, run Repair Disk Permissions.
Nope. Everything else is fine. I thought it might have stalled for a moment, but no, I closed out the browser and discovered the background picture had changed. Went into the photos where background images live and didn't see where I may have added anything. Looked to see if anything showed up on the HD as a jpg, etc and nothing. So, I chose the photo that was the background and replaced it. Then I did run the Disk utility from the start up disk, and repaired disk permissions ... and I'm running SOPHOS right now (we had a power outage this morning and I had to start all over again.) No stalls, no slowdowns... everything "seems" fine.
Why wouldn't SOPHOS find the malware? I thought that is what it does?
Should I uninstall and install AVAST?
Ah - you're guessing that there _might_ be malware now?
And all you have to go on, is the changed background image?
More likely your power outage caused a minor file corruption, with the result of randomly choosing some other background.
Some weirdness after a power outage is not unusual.
reset your user file ownership in your terminal:
sudo chown -R myusername /Users/myusername
Change "myusername" to your actual user account name - in both places.
That sudo will ask for your admin password. Don't be nervous when you don't see your password as you type. That's a security feature in the terminal.
You'll get a prompt when the command completes. Then, restart your Mac.