There is an add on for firefox 4 that allows you to capture the web page you are on and annotate ,blur sensitive info and all that neat stuff. then you can save the annotated version to local disk or online .
Sounds a little like greenshot, www.getgreenshot.org.use (http://www.getgreenshot.org.use). It's very easy, quick, simple to use. It allows you to do as you've said plus you can select only a certain part of the screen, add highlights, pointing arrows, circles, text boxes, all very fast and it saves the new pic (or screenshot) to a file you've setup. I love it, but haven't tried what you've suggested.