I had the very same problem for a long time and your post is the only evidence I've found anywhere that someone else has experienced this. Hopefully you've solved it by now, but if not, do you happen to have Adobe Acrobat installed?
For me - I had the same problem, I could only see images in PowerPoint in thumbnails or presentation view. My workaround was dual monitors, leaving the presentation on one, and fumbling around on the other and looking over to see what changed. I reinstalled lots of stuff but nothing worked.
Then I finally tried this: Add or Remove Programs > Adobe Acrobat 8 Professional > Modify > X (i.e., set to "This feature will not be available") out Acrobat PDFMaker.
I need that tool (I use it to create a PDF from a Word Document while keeping all internal hyperlinks (e.g., table of contents, references) functional - something which cannot be done through Print to PDF.) but use PowerPoint much more. I'll just reinstall/uninstall as needed.