Ummmm, goto the persons profile, click in the write something box, then underneath where you'd normally write stuff there are some buttons, hover over them and one say link, click that and add the link to the pic there.
if it's a pic from google images you need to click it to expand it, then right click the expanded pic and select "view picture" then copy the link from the address bar.
The new look google images is a pain for this sort of stuff!