You should paste in the url right there where it asks you, if I think the window you get is what I think it is. . . .
In the "Add Reply" window there's that image of a picture, select that, a window pops up with a place to past the url, paste it, and when the post is launched you should have an image.
Or. . . in the body of the post type This is the way to manually do it with code.