I would assume this is done by writing [img] *URL location of the image you want for the link*[/img] without the spaces and with actual URLs.
Let me test that out.
Warning: if it works, the link will take you away from AG. But click it if you wanna see my drawings.
Okay, that code is just mocking me now...
[ url= *URL to (for instance) your profile here* ] [img] *URL location of the image you want for the link* [/img] [/ url]
Hopefully those extra spaces kept it from linking. Otherwise hold your cursor over it or something.