How to cite a source that has vanished due to link rot? My colleague is writing a manuscript about a subject that is rarely reported in the mainstream literature (predation events). A few years ago on a fishing forum she found a photograph that demonstrated this phenomenon pretty well and saved it to her computer, but unfortunately between then and now the original source has vanished: Google Reverse Image Search can't locate it, the website is no longer running and is also unavailable in the Internet Archive. The original URL to the photo was also not retained.

What's the proper way to cite this source? It's not critical to include but it would definitely improve the final manuscript.
Before worrying about the citation your colleague needs to gain permission to use the image in her work. She needs to contact the owner of the image and ask them for permission. The image could then be used and the citation could be a "personal communication" or the URL of where the image was taken from with the access date.

Finding the contact details of the image owner will undoubtedly be difficult, especially given that the image is not actually on the forum anymore. But if the forum is still active perhaps a question could be posted on there (with the image) asking if anyone knows who took the photo / how they can be contacted and perhaps she'll get lucky.

Edit: As pointed out by StrongBad if your colleague is not planning on actually reproducing the image in the document then seeking permission for its use is not required. If she just wants to describe the image, I guess the only option would be to cite it using the URL where it was, with the access date. However, given that the reader could not then actually find the image themself and look at what your colleague was describing, it probably wouldn't provide that much useful information to the manuscript.