I’m not an expert on this, but the Wikimedia terms of service would be binding on the contributor rather than the original artist.
Commons has a specific template for indicating that the photographer wants a particular form of attribution under CC BY-SA 2.0. If you see a request like that, you should heed it, just in case. There was a high-profile case in which a photographer started suing people for following that request but with slightly different punctuation.
Newer versions of the license make it harder for authors to go after reusers on technical grounds. Creative Commons publishes a best practices guide for attribution that provide a safe harbor in a similar manner as OSMF’s attribution guidelines.
Regardless, the onebox template squishes the image into such an inscrutably small thumbnail that it would be much easier to argue that it’s fair use. If you link to the original image for the purpose of showing the image at a more comfortable size, you’d want to pay closer attention to attribution requirements.