Haven't noticed this when sending mail. I know if you receive a mail that has a mail attachment it simply means that rather than copying the text/images the sender just saves the mail THEY have received and attach it as a mail attachment to yours. A rather poor/lazy manner of mailing, unless you are actually meant to see the original mail.
If you mean why you get the piccy in the mail and also as an attachment, the sender would have drag-n-dropped the image in the mail rather than pasting it.