Without getting too bogged down in the "Image Credits" proposal, which as Gregory Maxwell points out is somewhat of an orthogonal issue, I'd like to propose Keeping it Simple and looking just at the image linking part of the idea, highlighting the distinction Elliott Cable pointed out between icons and images.
There's been no shortage of suggestions for syntax, so I won't rehash those here. The main objection is the loss or hiding of the copyright and other information to be found attached to the larger image. But proposals to try to make the link(s) go both ways quickly descend into a morass of complexities.
I suggest we do allow the optional linking of images to other content, with the following proviso: doing so is acceptable only the image is used as a navigational icon. Typically it will either be a drawn image (a button), or a thumbnail photo. So I would propose this formal specification of the proviso: "navigational" linking is acceptable if:
1. The image is 100x100 px or smaller, and ( 2a. the image is PD, or 2b. the same image also appears on the linked-to page and, there, has the old behavior of linking to the full-size image plus its metainformation. )
Whether to try to enforce such a requirement by policy or in software is an interesting next question.