Problems with updating my screen (2)

The last time I updated my iMac 27 I had to shut it off and unplug it as I was going to be away for a unknown period. When I returned, reinstalled the plug in the back of my iMac 27 (late 2011) the image on my secondary screen was one of your stock ones, it's the snowy mountain in the fall on a lake. I tried to change it several times with no luck.

Today, for no reason my Mac screen changed to the same image. I didn't put either one of them up and I have failed in every way I could to change the image. I need help, and I'm sure it's just one or two key strokes and all will be fixed.

Cheers.....Me