graeme_p - 1:03 pm on Jun 20, 2011 (gmt 0)
Given that they appear to have implicitly agreed to your usage of the images, you may have defences like equitable estoppel. You need a lawyer to look at the exact facts (the communications made, what reasonable expectations you had, etc.) to establish this.
Most people here are not lawyers, we do not know all the facts, and we do not even know what jurisdiction you are in!