I've answered a question on the intuition behind the Arc Length formula.
I would really like to add a couple of simple pictures to illustrate the two distance computations. They would involve a simple curved line, a straight line, and some labels.
Locally (i.e., at my laptop at home), the best I can do is use Paint, and I'm very unhappy with the results. Alternatively, I can produce pictures using LaTeX's picture environment (I'm reasonably good at it) and then create PDFs out of them, but they tend to be page-sized, which is overkill here.
I know I've seen several people illustrate their responses with small simple graphs, and if someone could point me in the right direction I would appreciate it.