Can't we ask this publisher for a small insert to promote Commons as an exchange!? With all the money they will save using our pictures, I think it's a good deal for them.
Maybe they could add an Appendix telling that images mentioning Wikimedia Commons are free and can be found at http://commons.wikimedia.org/ With a summarty of principes more or less extens depending of the space available?
Hmm, if they use really that many commons pics and every author requested "Wikimedia Commons" to appear in the credits, then this seems to be enough promotion. This will surely attract attention of the interested reader. An extra appendix would be an overstatement of the importance IMO. Do you see what I'm saying? If the mention of "Wikimedia Commons" among all image credits doesn't stand out by number then Commons apparently was not an important source of images. On the other hand if it does appear that often then the appendix seems unnecessary (maybe an URL would be nice though).