Well that is bizarre. When I search for your 1st or 2nd pic on Yandex the new photo does
not show up in similar images. It doesn't appear anywhere. Very strange.
I didn't try Google though. I avoid using them at all costs. That was what I missed. Nice work.
I use TinEye and Yandex for reverse image searches but I don't stop there. Using the results, I take the URLs and the thumbnails and do all kinds of partial text searches and combination image + partial text searches in Yandex. This will sometimes reveal mirror galleries (which aren't dead) or results that don't show up with just a reverse image search alone. I then take those results and repeat the whole process. Sometimes it leads to new things. Sometimes it doesn't.
I also did reverse image searches on every image in that imgbox gallery I found. Like yourself, I noticed they'd been posted together in several places. Searching for the other images lead me to other more complete galleries, one of which showed a thumbnail of the new image. I then used the thumbnail to search (but I didn't try Google!

).
I didn't find the old forum connection directly. I found it by back-tracking from an image link result (that was dead). Searching for the image link (i.e. a partial URL search) revealed it had been posted on an old long dead forum. It was about 4 levels down in my recursive searching.