Alex beat me to it. Right. Sorry, it's not the WRITE command. It's this:

Var x = load("http://xxx.xxx.xxx/servlet/?POST?_PNGJBIN_")

You  will get a return from your server, but in the process Jmol will send the PNGJ file in the  POST. I can send you the PHP side of this if you want to see it.

Bob

It would be very valuable for me to have a look at the PHP code. I've made several trials some time ago, but could not find where the data is in $_POST 
Paul