If you are using a Mac (as opposed to a Windows) PC, you may be interested in a program from Plasq called Skitch.
Obviously based on the word ‘draw’ (just joking) it allows users to import bits of the desktop or browser window, or photos from iphotobooth, or from the hard disk, and then sketch on them.
There is a pencil tool with various thicknesses of point and various colors; shapes, text, and other tools. And then you can save the finished product on disk or send it somewhere.
It is not a replacement for photoshop; and Macs have Grab built in, with which one can grab bits of the desktop/browser window – but Skitch is worth exploring a bit more, which I shall do.