I'm afraid you can only do it if the image is somewhere else online - you can't upload an image from your own computer.
Not direct, BUT you can do so via a third party hosting site. I use
Lunapic. I prefer it to image attachment, which I can't do anyway now that my sub has lapsed. My bad for not renewing.
@hernibs: if you want help with Lunapic, let me know.