The world would've been a way better place if it was filled with lesbians breastfeeding each other.
Seikon no Qwaser episode 7.
It's probably gotta be tribadism. There's nothing strictly yuri about breastfeeding (or cunnilingus, fingering, etc.). Tribadism is something that can only happen between two girls, like yuri itself.
I've seen that before, but I thought it was called scissoring.
And the others versions of the first pic.