Getting traffic to a image based site

Ok dudes, I have an image based site and I need to increase the traffic.

Any ideas?

Please share even though they might seem obvious please share any method or ideas.

At the moment I just do some on page SEO and rely on google image search and organic listings. I get around 1,500 unique visits a day.

I am thinking of backlinking to the category pages to boost those in the search. But any other ideas? A building a large facebook page would be good but I never have any luck building fb pages.