How do I export a .MOV frame as a PNG?

Maybe you could try taking the one frame and exporting as an Image Sequence. I believe those file are indeed true .png,.gif, etc.

Yep. Pretty annoying.
Movie to Picture creates a .pct file. Consider it a "wrapper" just like .mov extension.
If you set PNG as the compression type you can open the .pct file with most image editing apps or even Picture Viewer and save it with the correct extension (no additional compression).
I just did a test on my Mac and just edited the file extension (.pct becomes .png) and that also worked.
Did you know that you can export at dimensions larger than the source? Just Control-2 (Double size) or drag the grow window and your export to .pct will be at those larger dimensions.

I wonder if the .pct is just a bit of fluff and that the underlying file is really a .png requiring only an extension change. Guess I'll have to reboot into Windows to find out 🙂

It doesn't seem to be a simple as simply changing the extension.

I ran into this problem when trying to masquerade a QT pct file as a png file as a preview for a flash video. When embedded in the flash player, it would not appear in any browser. (BTW --- It appeared just fine as a plain img in any browser.)

Here was my rather specific fix, which shows that simply changing extensions doesn't always fool every application:

1. Open the png file in a text editor (in my case, emacs) which will let you edit binaries as ASCII text.
2. Look through the file. Assuming that non-ASCII characters are shown in octal with a preceding \, you'll see the following
\377\377\211PNG
3. Delete everything until just after the two \377 characters, so that the file starts with \211PNG. (If you are editing in hex, look for ff ff 89 50, and delete up through the ff ff.)
4. Save the file.

After doing this, it embedded just fine.

So.... simply swapping the pct extension for png isn't a true fix in all ways. I'm guessing that pulling off the start of the file tears off the wrapper alluded to above.