Thanks, narfstar, I have just barely looked at that site so far.
Yes, credits turn out to be awfully complex, partly due to the setup of my project (four artists in one panel), but I keep thinking about how to make things as complete as possible. Tilliban may be right in saying I shouldn't overdo it. I'm starting to figure out that there may be limits to the credits I can compile. Mostly uncredited artists is what bothers me. Right now, at least a few of the uncredited artists I've used are either clearly identified on a JVJ card or else just pitifully easy to recognize. I'm a little disorganized, I'm new at artist spotting, and also I have little knowledge of the artists, publishers, etc that are discussed here. I'll get better at all this.
So "plundering" the public domain brings up difficult questions even though there's no connection to law. There's no obligation to include any sort of credits, but for my project, which uses dozens of artists, I feel like there should be credits. In each panel, I try to preserve the integrity of the art even with considerable alterations being made, and I always wonder what the artist would think of their work being reused and altered. It's mostly forgotten names and forgotten books in a disposable media, but to me, it's like chopping out pieces of amazing canvases in a giant museum. I can draw, but not as well as these artists. So I try to credit book, page, year, and artist. And every last fragment used is individually linked in each page's credits to the DCM source page. I don't think every project that uses public domain material should list credits, but for my project it seems really appropriate, and partly the credit links are just a fun thing -- you can see an object named in the credits, and click the link to see the source art. Hopefully that's amusing for a few people, and one more way to direct attention to the DCM.