This is the result when copying a pic from Finder to Firefox 22, only a placeholder was uploaded:Pasting images
If you use Chrome or Firefox and have an image in your clipboard, you can simply paste it into the editor. It will automatically be uploaded as an attachment if possible. If the upload is successful, this works exactly like uploading an image and then clicking the "full image" button.
Note that in Chrome, the image data must be in your clipboard (such as from pressing print screen). In Firefox, you can either have image data or you can copy a file that's an image and paste that.