I had a nice background, a photo from the internet. It had been there for quite awhile. Then I tried to set it to a different one, another picture from the internet, by right-clicking and doing "Set as desktop background...", but it instead set my background to blue. And now I can't change it.

I also tried downloading this photo (and a number of others, all jpegs) but when I tried to open them to set as background directly from the downloaded file, it just said it couldn't, because the file was damaged or had an unrecognized format. JPEG?? Now the Mac can't open JPEGS?