hmmm Ive just been doing a little bit of googling re re mshtml.dll and IE8
From here -
Whats wrong with the mshtml dll in ie8?.
Interesting that someone else has noticed that mshtml.dll isnt working properly on his version of IE8 and it had the effect of not being able to view folder background images (jpgs?)
The more Ive looked into this, more and more I see mention of IE8 incorrectly registering some .dlls and I've also seen mention of it on occasions stopping some images on the local machine that rely on other MS related software from working.
The symptoms are very similar to yours in that the image can be opened standalone but not via the other MS applications such as explorer backgound images, visual search.. and presumably this would also include OE. :/
Unfortunately though I dont seem to have stumbled on a solution.