Did you mean to post the thumbnail version of the picture. I don't use webshots, but I believe you will find size selection boxes across the top of the pane from which you copy the URL for the picture. Select the size first, then copy the URL.
You are posting in a picture thread, but I don't think you are trying to add a picture. If I'm wrong, let me know and I can help.
Note: to check your signature, you don't need to make another post. If you will notice, that after your last change, your first post in this thread has the same signature as your last post.
As it turns out, all of your posts that display signatures, display exactly the same signature, even after you make changes. That is because signatures are not stored with the posts, but are added from your account when the display engine creates a page.