Lefteye has posted an excellent tutorial here dprevived.com/t/how-to-embed-an-image-via-an-external-address-full-size-clickable/1090/
They key to it is to manually input the code you need. So, you need to put in the URL code and the IMG code as follows:
It looks like this being spelled out: you put open square brackets, "url", equals sign, the url address of your image, close square brackets. but doing "img" is different. You have Open square brackets, "img", close square brackets, the url of your m=image again, then open square brackets, "/img", close square brackets, open square brackets, "/url", close square brackets. Like this:
[url=address][img]address[/img][/url]
If this all looks like kindergarten stuff, I apologise. But I misread Lefteye's post to start with and had to go back and read it really closely to work it out.
Alan