basically what >>3 said

it is only worth bothering if you do something unique and interesting with your imageboard that isnt currently being done. don't bother starting a 4chan clone, there is already 4chan for that, your site will die on its arse.

i started a successful imageboard that is still successful today (although it changed hands a good few years ago) but it only became successful because a) it was entirely dedicated to something that 4chan does not offer and b) it came about when 4chan and the english-language imageboard phenomenon was still in its infancy so competition wasn't quite so established. i gave it a tiny bit of promotion on 4chan and a forum dedicated to its subject and it just built from there.