There doesn't seem to be a court case that defined if images should count as part of the work or not when talking about licenses. Many AGPL projects clarify their intention and exclude css, images and styling regarding agpl compliance. At this point it could almost be said to be a industry practice.
Outside software, there doesn't seem to be a clear legal precedence either. I commonly see news paper include CC-share-alike images in articles without adding the CC license to the article, website or the news paper itself. The question about copyright almost exclusively end up being about distribution rights of the work in isolation, or the right of the author to have the work associated in a specific context.
Is that tested in court? Because that was one of the scenarios described as potentially problematic before to me by a lawyer (Applying a corporate theme to an AGPL system, breaching the contract with the designer of the theme that doesn't allow you to re-license the design, only to use it)