Thanks for the replies, Riley and Thomas.
Actually it's not even books I'm working with--just single documents. But unfortunately the graphics folders and subfolders need to be in a specific format, so I don't think the Archive plug-in would do much for me.
However, Riley's suggestion to Add > List of > References in a book reminds me that you can also generate various kinds of lists in documents, not just TOCs and indexes. I remember now that you can generate a list of references, so presumably that can be configured as graphics references only. I'm not on my work computer now but will try it later.
Cheers,
Joe