Images Appear in FCKeditor, but disappear when you save the post.

I was happily creating a new post. I enter an URL to an image that is already on my site. I see it appear in my editting window (cool, i'm new to FCKeditor). However when I save my post and view them, the images are MIA?!?!?!. I view the source and they are missing. I re-edit the post , and I can see the images exactly where I placed the URL.

Any ideas ?