What is proper acknowledgment for figure authorship in a book chapter? A senior researcher, with whom I worked a few years ago, is writing a book chapter. He contacted me to ask what was the latest work of our group on the topic of his chapter. I gave him a few links to recent articles and the preprint of an article soon to be published. He followed up by asking if “[you] would have a figure to illustrate [topic of the preprint]… preferably something that does not require copyright authorization paperwork”.

At first, I thought that was a bit much to ask… I had never asked for figures from anybody who was not an author on the paper. But the situation may be different for a book chapter, and obviously I'm glad to share the news of our most recent results. So, I took an hour tonight to make a nice illustrative figure, and am about to send it. However, I'd like to make certain in my mail that I ask for some sort of acknowledgment. So, my question is: if someone designed a figure for a book chapter, how would that person be acknowledged? in the figure caption, e.g. “figure courtesy of X”? in another way?
In the acknowledgements sections, the authors could write:
"The authors would like to thank F'x for providing us with Figure X".

Alternatively, in the caption of the figure they could write "Figure courtesy of F'x."

Both are acceptable, as long as you are happy. I don't think they are obliged to thank you in the paper at all.