Usually refers, nowadays, to loose women. Commonly, people will nickname a woman a "bucket", if she sleeps around, or has a reputation as a slut, or a chicken head.
"Ew, Christy? Hell no, I won't touch her...she's a bucket, for sure..."
Basically, "nudies", is a cute term that refers to nude pictures. I would imagine someone was just too scared to say "nude/naked", so instead, substituted those words, for a more, pop, PG-13 rated version.