Desktop background ?

I have a problem that just appeared out of the blue and has me totally
confused. For some reason, the last few days my desktop background has been
stretching every image I have used previously - even those that are less
than the regular 800x600 resolution. Note that they appear in the preview
screen correctly, as in the correct size with space around, but now ALL are
stretched to take up the entire screen. Also, I thought there were three
options on the background display ... tile, center, and stretch. Now there
is no stretch radio button. Those previous images (BMP's) that I set to
leave room around the perimeter so my icons fit there are now stretched to
the extent they are distorted.

Does anyone have any ideas what is going on and how I might get this back
to how it was justa few days ago?

I use a number of malware/spyware/etc programs as well as 2 different
scanners. I get no reported problems running Sandra. I'm stumped.