I've been using Paint.Net for several months now and don't know how I got along without it (actually, I do know: poorly). It's fast, easy to use, reliable, and it fits my communication style.
What I like to do when communicating with folks is include a portion of a screen shot and highlight the area that I'm discussing. This can be circling a field on a form or shading the area. Some products are too big and bulky for this (read "PhotoShop" or "Gimp", for example, both of which I use) and some don't have the simple drawing tools (read the Microsoft built-in tools or most of the basic image editors available).
Get Paint.Net at www.getpaint.net. I think you'll enjoy it!