Recently they added drag-and-drop file attachments, and now they've extended that ability to image insertion. Need to remind someone it's peanut butter jelly time? Grab your favorite dancing banana and pull it in to the message window! You'll see a brief animation while your image is uploaded and processed.
Both JPEG and PNG are supported, though you can only drag one image at a time -- at least in my testing. If you've managed to select and drop multiple images, let us know in the comments!