Turn on 'Inserting images' in Gmail Labs. And while you're there try 'Message Sneak Peek' too..
I don't think that addresses the essence of SlackerIII's comment. You still have to go through the tedium of launching an external program to take the screenshot, choosing a filename/save location, and then inserting/pasting into Gmail. See my comment (up) about Outlook 2010.
With the python FUSE bindings you could setup a .jpg file on your desktop that could dynamically change based on the contents of your clipboard (and could disappear when the contents aren't an image). It wouldn't be too much more than plugging a bunch of libraries together.
edit: (missed the mention of Outlook, guess you aren't able to run FUSE)
If you have Firefox, you can use the Screengrab! addon to copy the screenshot to the clipboard.
EDIT: Only webpage screenshot, not full desktop. Should be easy to implement an external (background) running program to copy to clipboard the screenshot, though.
Before the lab even came out, you could "paste" image data into an html email.