If you uploaded it to your server (as you said), you don't use the URL (i.e.
http://www.yourdomain.com/imagename.ext) - you should save it in any folder within or at your document root and insert it
as instructed here.
If you meant an image from a remote server, that's the ONLY situation you need the URL, and if true:
Through the GUI insert->image I don't recall there is a way. I am pretty sure you'll need to insert any image from your hard disk then in design view click on the image (assuming "Commands -> Display External Files is checked) and then in the properties panel at the bottom in the SRC box delete the value there and replace it with the full URL. Do not paste it, type it in manually.
BTW...
What you're doing is called hotlinking and it is frowned upon greatly in the development world. You're using someone else's bandwidth to display the image, not to mention they can block you from hotlinking and/or if they change the image you have no control over it.
So what you should be doing is, assuming the image is not copyright or you have permission from the remote site, right click/save as and save the image to your disk's document root or maybe in an images folder inside that then insert via the GUI. In production, it'll load much faster plus you have full control over the image.
No need to reply with "They said it's ok for me to use the image" - this is informational only, not a law or something!
-jim
