I couldn't help noticing that hosting homework questions from textbooks borders on copyright infringement. Especially when pictures of diagrams are used. Is/should there be a policy promoting citing the textbook of origin, Chapter/section and question number? This might make it easier for others searching the internet for other problems in the same book as well.
As a matter of copyright/plagiarism, then yes, whenever material is quoted verbatim from a textbook, or from anywhere really, the source should be identified. I think that is mentioned in our homework policy, but if not, it should be added.
I would not expect this to be a "prosecutable" violation of copyright law, since quoting part or all of a homework problem for the purpose of getting help solving it probably falls under the fair use exception. (But I'm not a lawyer.)