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If the Google reverse image search doesn't find anything, it could be because the image is new and hasn't been indexed yet => wait a few days.
If the pic is from a website that Google doesn't index (e.g. Furaffinity), then your best hope is that someone will upload the pic somewhere else and include the source.
Furaffinity doesn't let you search by MD5 or width/height or filesize or any other useful parameters (because FA's search function really, really sucks), but sometimes you can find a pic if you pick the right keywords.

The only long-term solution is going to these websites and asking on their forums that they add MD5/width/height/filesize to their search options.

Not that the people running FA will care, of course.
I've already suggested this feature on the FA forum, and there was no reaction from the staff:
https://forums.furaffinity.net/threads/138459-FA-search-new-options

Munkelzahn said:
because FA's search function really, really sucks

I remember when it didn't even have a search function. The one they added hasn't really been much benefit though.