Images are not like videos or links. Videos and links can/do suffer from link-rot. Images, however can, and should, be hosted on SE's imgur host. So images will not suffer from the same sort of rot.
That said, image only posts should be judged by the same merits as any other posts. If an image is explanatory and gives all the relevant information, great. In fact, we have lots of image only or image driven answers that get right to the point. If someone asks where something is, sometimes a map or screenshot is enough to get the relevant information. It is probably not the best answer, but that doesn't make it an awful answer either.
The major caveat for me with images is accessibility. You need to make sure that the descriptive text is helpful in explaining the content of the image. It should allow someone who can't see the image, for one reason or another, to understand the contents and still get the answer to the question.
Now, about the flag in this case. Flags should be used when content is so low quality it can't be fixed or improved. A flag here seems more like a super downvote than trying to denote something that needs moderator attention. Remember, when you flag something, make sure it's something you think needs/warrants moderator attention. I know our mods are happy to help, but also have a lot of things to do as well as lives outside here. So just be mindful of that. A flag, like a close vote, is not a super downvote.
Long and short of it: Judge image-only answers by the same bar you would judge other posts. With the minor caveat that making sure it is accessible is extra important in this case.