Initially, Gmail employed a function that let users drag/drop images from their desktop right into the compose window. Now, with the latest version of Google Chrome, users can paste images right from their clipboard. For example, users can copy an image from the internet, email, etc. and paste it right into the email message. Click here for more info. 

*Currently, this only works in the Chrome browser. Google is working on enabling other browsers to have this function as well.