I don't know about you but I am sometimes guilty of putting images on here with "unknown" as the source. I try not to use them to see if I can come across the source but sometimes the image is too good to pass up for a specific post.
Not sure if anyone has come across this or not but I have recently stumbled upon Tin Eye, a reverse image search. This website has become a lifesaver for many of the older images I have collected over the years and forgot the source.
They won't find every single image you search for but there are tons that have come up when I put them in....they sometimes link to blogs that source the photo but may also link directly to the original source. I have even done old scanned in magazine images with success!
Try it out here. Hope you find some of those images that have long been elusive to you.
Image: Vida Zaher