How do you cite an image no longer found on the internet
If you can't find the source at all, you should choose a different image. You can't simply put a citation like:


  Image. Found on the internet, source unknown.


Even though it would be nice to be able to do that, that defeats the purpose of having a citation at all, which is so that people can see where you got it.

You might find the following questions useful:


This question that was pointed out in the comments seems to be a case similar to yours, with good advice. Basically you need to somehow track down the source; once you have tracked it down, the specific citation text is not such a big deal, as long as you are listing the source.
This question is also relevant. Unfortunately or not, the culture is extremely pervasive in academia to just go pasting images from the internet without citations (especially Google images, and especially in presentations rather than published papers). This is not completely ethical, but it is common practice.