Update — Those questions have been merged, and I posted an answer.
Since nobody has pointed out an existing one, I hereby declare the question
to be the canonical such question, and ideal target for duplicate flagging, henceforth.
I don't know about a canonical one, but here's one I wrote a few years ago, perhaps more suitable for an analysis student than a beginning calculus student. But the language is extremely plain and, I think, clear:
It does at least have the pictures showing a lower sum and an upper sum and why they will (normally) tend to converge as we refine the partition. In case someone else wants to use them, I hereby release the diagrams into the public domain.