You have to:
(1) Chose a subject that suits an online forum. One that not only generates discussion, but which typically involves media sharing (photos, video, etc). Be highly targeted... but avoid being so niche that you don't have enough people for your potential member pool.
(2) Be meaningfully different from what (if anything) is already out there.
(3) Have interesting content. Plan on posting a LOT of content the first year - which is why posting interesting photos helps. Post reviews, or guides, or FAQ's or whatever your members will find the most interesting.
(4) Give people a reason to return to your site on a regular basis.
I kid you not, on my site we have just started a 6 year contest that will have people posting updates and photos for six years. I didn't do it for the engagement - I did it because it makes sense for my site and my members - but still it is a reason for people to keep coming back.