There are a number of techniques I recommend to clients depending on specifics, but, while it may look good, in no case is a graphical link a good ranking tool. Because the engines do not appear to use the alt text of an image to influence Link Reputation, the image link serves to dilute the reputation of the targeted page. I would always make these nofollow links and provide a separate text link, or include text along with the image in the same link — that does appear to work — and then style it using CSS so it looks decent.