Are you running like 98 or ME? Usually, this means you have active desktop installed(which does drop your performace...) Go to display proporties/web, then disable active desktop. You should see your old pic. Go to background and select none.
Then, what I do is save my backgrounds as BMPs. They are larger, but easier for your comp to load and you don't have to use active desktop... However, if you REALLY want to use jpg images, then just do this, but select your previous background again. However, you will probably still get a solid color when you log off(when your desktop disappears and all...)
