Some image hosting sites like Imgur and min.us support image uploading via drag-and-drop in Chrome and Firefox. It would be nice if you could upload an image and add it to your post by dropping it on the edit page.
A few days ago (February 2011), a new image upload is being tested on Meta, adding support for paste and drag-and-drop:
However, this does not support direct paste or drag-and-drop into the edit page; one first needs to click the "Image" button (Ctrl+G) to show the upload dialog.
In June 2015, this new dialog was implemented: