To post an image, you first need to upload the image to site accessible from the Internet. For example, I use

Once it's uploaded, you then recover the link (URI) for it, then in the C9 editor, switch to the HTML view (see the two tabs below, where it says Design and HTML), then find the point where you want to paste the image, and paste in the actual HTML.

When you switch back to the Design view, you should be  able to see the image.

Anyway, here is a screenshot of the same thing in in VS2005 Pro (Click the image to see it full size):

And here are the Links to the appropriate documentation for [VS2008] and [VS2005]