I did check for existing messages on the subject. Since this feature worked in version 0.1, I considered it a bug. I searched through the K-Meleon Bugs forum for PNG and found nothing (try it: full-text, all dates, and you'll see this thread and nothing else).
For those who may be looking for this bug under Bugs, here are the following links I have found in the other forums:
"K-Meleon Improvement requests: this is important... PNG files" suggests that you upgrade K-Meleon with a Mozilla nightly build.
"General: Tuning KMeleon (using nightly build)" describes how to do it.