killall SystemUIServer
I can verify the same problem, it drives me crazy with Photoshop and screenshots. If I copy-and-paste or DRAG an image directly from Photoshop I get a gigantic Tiff, and if I shift-cntl-cmd-4 screenshot to clipboard, paste, same problem. (It creates something call pastedGraphic.tiff.) The email size is much larger -- check the size estimate at the bottom of the window as compared to a shift-cmd-4 screenshot to disk (jpg). Or try dragging the image from Mail to the desktop.
But it can get worse. Try taking a screenshot to the clipboard, paste it into an email, and then also open a coipy of the screenshot in Preview. Select all, copy, paste into Mail. A shot that was 176K on disk, 238K in Mail, becomes 1.8 MB. Similar problems arise from Photoshop.
Another hint: If you choose a size less than "larger" in the Mail "Image Size" selector, it changes TIFF's to JPEG. Now the pic is called pastedGraphic.jpg.
http://forums.macosxhints.com/archive/index.php/t-72700.html
in fairness some of these issue probably arise from the way other apps (including the screen grab) prepare images for the clipboard. Mail then accepts what it is given. HOWEVER we do want to fix the format to something acceptable for mailing. It is this a buried feature, issue for a 3rd party developer, or do we just wait?
Would love a fix.