I finally got a chance to upload the book posters to Flickr, with links to the original images that we adapted. All of our 'poster' blog posts include an image credit and link, but I wanted to include one at the source too.
(Still TBD: additional images used in our book trailers.)
And, here are two credits that are listed on the copyright page of the book (with the links spelled out), but haven't yet appeared on the blog:
The following 2 people were kind enough to make their photos available to all under a Creative Commons "by" attribution license, including taking the risk that adaptations do not do justice to the originals.
Thanks also to Flickr for providing a CC-BY search page.