Can't say I've picked any most of the time, just that other people have been watching them on the TV and I've seen them too thanks to this. Haven't seen any good ones yet, the only real exception has been Sailor Nothing.
Most artwork is in more vertical formats, though. For the MAL sig, it has to work as a wide stripe, and readability of the text should also be considered. Many otherwise good images do not work for this simply because of the text.