Well, you're going to need the full URL of the image, say http://26.media.tumblr.com/tumblr_lxvanp2fJa1qbq86qo1_500.png
What you do then is type <img src="http://26.media.tumblr.com/tumblr_lxvanp2fJa1qbq86qo1_500.png"> into your post; note, however, that when HTML is in a post the auto-formatting does not go into effect, so you'll have to manually format your text.
Noted. Thank you.