How to generate previews from flic.kr short URLs

On twitter you may see URLs of this kind: http://flic.kr/p/7ME9tR. Well, at least I saw them when coding TweetMountain (more specific when writing its image handling routines).

I needed to get the actual preview image from those URLs. After a little research I found this page (esp. the post by kellan)...

The URL just needs a little rewrite to get the image previews. This is what kellan wrote:
flic.kr/p/7sqMtA (this is the URL)

This has to be rewritten to:
flic.kr/p/img/7sqMtA.jpg
flic.kr/p/img/7sqMtA_s.jpg
flic.kr/p/img/7sqMtA_t.jpg
flic.kr/p/img/7sqMtA_m.jpg

Generalized form:
flic.kr/p/img/$shortid_[stm].jpg