A friend of mine ran into this a while back. What happened was a website you visited installed a plugin/object into IE
that makes all images bitmaps. These things are intended to make it more annoying for people to save images on
websites. However, being punks they put/have no limit on when it runs and where, so it remains active until you
manually get rid of it. You can go into the internet options and delete the temporary internet files, and you probably
have to remove it from the plugin/object manager as well. It's somewhat easy to find info on this with
google.
But seriously, consider using Firefox for web browsing. This is one of the least malicious exploits that IE allows;
it's not safe to browse with it.