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If I get you correctly, you want to go "up" one directory and then "down" into the directory structure from there, correct? If so, you can use ../ to go "up" the directory tree by one folder. Using 2 ../../ goes up two directories, and so on.
So in your example, I think the relative expression would be "../../cotter/license.html" -- since you say that /cotter/ is actually a level above /license/ and not parallel (on the same level) which would be "../cotter/license.html"
/images/top/file.gif
This will tell the server to go to its root and then down through the images and top directories. Writing this is the same as writing "www.example.com/images/top/file.gif".
Caveat: If you are using a hosting service, it is worth checking whether this is supported...