It's done the same way that you do with a normal text or image link. Simply insert the URL tags around the IMG tags.
Code:
[url="www.google.com"][img]http://www.google.com/image.png[/img][/url]
That should do it. Just check the formatting on the URL and IMG tags. I never cared much for BBCode, so I don't remember if you need the quotes around the URL, or if the img should actually be img=.. but I think that's right.
_________________latina mortua lingua est // ud mortua ud ea possit
prima meas ancestras necavit // et nunc me necat