The words “script prompt” probably shouldn’t be there – it adds confusion and we’ll look into getting that fixed.
This box is to add an image to the body of your mail – the only way you can do that is to ad an image which is online – and has a URL associated. If you would like to send a photo that is located on your computer, you’ll need to add it as an attachment (which you should be able to see at the bottom of the mail screen).