I remember having this issue in the past with previous versions as well but recently it started here again. Whenever I click a link that links directly to a PNG image I get a download dialog; the browser won't display it on its own. Inline PNGs are fine, though.
ETA: It apparently only happens from PNG images linked from RSS feeds. Maybe it has always been so, I can't really say.
ETA2: IE 8 it is, as should be common sense but apparently it needs to be stated explicitly sigh.
Any ideas what causes this? It did work in the past.
Preemptive warning: I'm not looking for advice to change my browser; I'm looking for a solution to a specific problem with one piece of software. Just ignore the fact that it may have religious importance.