If you are paying for this advertising, then Google says you should use a rel="nofollow" tag in the link. This will negate any possible SEO benefit for you, leaving the traffic benefit only. If this image is being used because someone really digs your site, it still "might be" looked at as a paid link. In some cases, the value of a paid link not only gets negated but also triggers a penalty.
That said, in a linked image that Google does not suspect as being paid, the alt attribute's value is used to pass link juice for those words, although at quite a lower level than pure anchor text. You asked about "stuffing" - that can kill the value of the alt text, too. The alt attribute's text should represent what text on the image says, so that assistive technologies such as screen readers can report that information to their users.