Member Rara Avis
Try reading this entry, Roniece, and see if it answers your question? Essentially, the image you want to post has to exist somewhere on the Internet, with an address that allows you to get to it. With that address, you can allow access to it from your post, too.
For an example, click on the Edit icon on this post. You won't be able to actually edit my post, of course, but you will be able to see exactly what I typed in to create it.