Gmail just picked up another drag-and-drop feature, less than a month after giving attachments that convenience. Starting in Chrome, and coming to other browsers soon, you can drag pictures into messages to embed them inside the body.
You don't even need the "Inserting Images" lab enabled for drag-and-drop picture insertion to work. Simply drag an image to the spot where you want it, choose an image size, if needed, then finish up and send it on its way. Neat stuff, and hopefully coming to at least Firefox very soon.
Drag images into messages [Official Gmail Blog]