About every other post of assassin_(fate/zero) is the female variation. Doing a search with ponytail or jewelry doesn't get all the images with her, and it would clash if anyone else in the image has such. It's difficult to do an exclusive search on her.
She is a bit distinctive from the other assassins variations. It would still be implicated to assassin_(fate/zero). Any sort of tag name similar to the one I suggest would work fine.
Updated by user 11314