The Web is a mass medium comprised of niches, formed around psychographic preferences and affinities
Just because you build it, doesn't mean they'll come (or stay)...a few things to consider:
Your brand is probably not something people will want to form a long-lasting community around. What's the bigger idea?
Note: Consider the concepts or values related to your brand and the psychographic preferences they feed into
Define clear objectives for your community.
Note: "so people can upload pictures and share their stories and stuff" is NOT an objective....it may be a result of an objective.
Consider the value-proposition for users.
Note: again, "so they can post pictures and share their stories" is NOT enough of a value-prop. Why? because attention is scarce, they can do that anywhere and data portability is not yet a complete reality (yet).
Plan how to be useful (and compelling).
Note: your technology platform is not the first thing that should come to mind here. Your users' needs are.
Note: You don't just make friends and drop them after you've gotten all you want out of them (or maybe you do, in which case you should get off the internet and take a long, hard look in the mirror), you build relationships. Assess the human resources you'll need to be there for the community-- to support and invigorate it.
....I know I said don't focus on the technology first...but once you have the strategic approach down, "design for community" points to consider: