Hi Bastien,

Given an image file name, how do you programmatically
retrieve the permalink?

E.g. given Kohila_vesiveski2.jpg I can easily infer
http://commons.wikimedia.org/wiki/File:Kohila_vesiveski2.jpg
but not

http://upload.wikimedia.org/wikipedia/commons/thumb/2/24/Kohila_vesiveski2.jpg/320px-Kohila_vesiveski2.jpg

Where does the "/2/24/" comes from?  (The rest of the URL seems
to follow a regular pattern.)

On wiki, you may use [[Special:FilePath]]
<https://commons.wikimedia.org/w/index.php?title=Special%3AFilePath&file=Kohila+vesiveski2.jpg&width=800>

Using the API, I think imageinfo is what you are looking for (but there might be a better way):
<https://commons.wikimedia.org/wiki/Special:ApiSandbox#action=query&prop=imageinfo&format=json&iiprop=url&iilimit=1&titles=File%3AKohila_vesiveski2.jpg>