There is a great web service called Aviary.
Often while out on the web you come across and image or page you want to grab and also annotate. Aviary will do precisely this on the fly and allow you to save the completed image to you desktop.

To use it simply enter aviary.com in front of the URL  so for example you were at http://yahoo.com click in you address bar and change it to http://aviary.com/yahoo.com