Skitch

Skitch lets you quickly snap images of what you see on your computer screen and share those images via e-mail or the web.

Instead of typing a description of what you see, you can take a snapshot of your screen, annotate it

Writing and drawing float on top of the screen snap, so you can move it around

If you want to put your screen snapshot onoiine, you can use Webpost which is part of the Skitch progra. Skitch webpost has options for embedding screen snap online.