Just mentioning for those looking for sources:
IQDB.org will take an image you submit and look for it or similar on danbooru/gelbooru/sankaku/konachan/etc, its fairly narrow so it has trouble with cropped images.
saucenao.com searches pixiv, and catalogues for anime and manga/magazine/publications, it also has trouble with cropped images.
Google also has an image similarity engine, just click on the camera icon in google images and upload the image you want to find, it naturally has the best results, and does well with cropped images.
tineye is somewhat made obsolete by google, but I keep it in my options just in case.
e-hentai also has an image search function, if you're looking to source doujins or some manga.
I hope people will pay more attention to this, it would be nice it source requests could be kept to the things that really are hard to locate.
