If you have the image on the web somewhere, type the URL here, or copy and paste it if there's a lot of it, highlight and click the img box above the typing box, and it should link automatically.
A few "do-nots" though:-
1) No spaces in the URL - replace each space with %20 to save changing the image name.
2) No private sites - if you need a password, we can't get in (eg other forums and some photo-sites).
I typed this somewhere else already, but I can't find it, so no surprise you can't either! Right-click on the webpage image and look at Properties to see the URL for the picture you want to link to. This one is at http://www.hallvw.clara.co.uk/fbklnl.gif