The image will show it's from you, but offers no proof.
With PGP, and the free variety of it, GnuPG (or GPG for short), you can encrypt your email so make sure that no-one else except the recipient can read it. Another function it offers is to be able to sign your email digitally, which is totally secure proof that it's from you. It requires your recipient to also have GPG or some other PGP-compatible product, but as long as they do, they can very easily confirm that it's actually from you, and not someone masquerading as you.
http://en.wikipedia.org/wiki/Pretty_Good_Priva cy