Although I usually make my own art, I belive this will be a super useful feature, especially because World Anvil always encourages users to credit artists all the time. Although I guess you could put this in the "about" section of the book, but this would encourage more people to actually add it by giving them a slot for it in a less cluttered place. Thanks for suggesting this! God bless you.
In response to the previous comment, almost everything *could* be posted into another generic field, the question is whether it is important enough to be desirable to prompt for entry and show it in a standard way. I'd argue image credits on a site explicitly designed for artisitic creation very much fall into that category.
I think you can very easily put it on the first page of your manuscript no need for an extra field.