Okay, I think it's important to explain this problem in better detail, because I am experiencing the same issue!
Everyone who answers this type of question has the same answer: go into Control Panel or Quicktime Preferences and change the file association, but the problem is that pngs are not associated with Quicktime in either case.
This problem does not appear to be a problem with file type program associations. This problem is isolated to .png files viewed within Internet Explorer only.
I have tried to go into Control Panel to uninstall the Quicktime plug-in, but all I can find is Quicktime itself. There do not appear to be symptoms or resolutions outside of Internet Explorer itself. The usual resolutions do not work for Internet Explorer either. If I go into "Manage add-ons" in Internet Explorer and disable the add on, the png files are unviewable.
To be clear, the reason why Quicktime controlling .png files in Internet Explorer is a problem is because when I click on a link that leads to a .png image by itself (example: http://www.website.com/pictures/image.png), the image appears in the center of the window (rather than on the left side) and is stuck in a "fit to screen" setting, meaning that I can't enlarge it to its full size. Sometimes the image simply fails to load, showing a broken Quicktime link instead.