Hey David,

Not being an expert, the best I can tell you is to check this link :

http://www.pymolwiki.org/index.php/Png

Basically, first ray trace your image with the ray command setting the size:

ray 1200, 1200

Then export it to PNG with the png command setting the dpi in the process:

png myfile.png, dpi=300

This should make the trick I guess.

Best regards,

João