Very nice.

Care for some unsolicited hlint-like suggestions?

Why not simply this:

    img <- readImage "file.png" 
    -- instead of this?
    t <- readImage "file.png"
    let img = t

Why not this:

    map (y!!) x  
    foldr ((:).(y!!)) []　x　-- instead of this?

Note that a way of reading fold g [] is that replaces the list constructor (:) (aka cons) with g. So (:) as argument of fold is probably not idiomatic, since foldr (:) [] is the same as id.