But really, I don't think anyone knows. Has an imageboard ever successfully grown because of advertising?

To reply again, you can pay for advertisements on 4chan I think, and some imageboards have a likeminded 'webring' thing going on where they link to each other. It takes time to build up some camaraderie and find common ground, though.
There are also people that maintain lists of imageboards, but be forewarned that said sites are used by malicious spammers as well.
I think the best way to promote a board is basic word of mouth. The "COME POST ON MY BOARD" thing generally doesn't work unless there's an event going on forcing a migration, like the 4chan outtage earlier this year.
Unfortunately building a community site from scratch is very difficult if you don't already have the community.