Personally I would turn off the ability for lurkers to see the pictures, etc. Why would they register if they are getting everything anyways? Instead the non members should be allowed to see thread titles, etc. But when they click in, instead of the pictures they should get a message that they are not members and only members have access to the pictures and more.

Give them a reason to sign up. That to me is the problem right now.

Done! Any other advice?

Well only contents aint going to start discussions automatically, start discussions by creating dummy accounts, and once people start participating they will automatically get in the habit of starting new discussions.

Or create a usergoup that has differnt permissions ofr different ammount of posts
10 posts - able to view pitcures
20 posts - able to upload pic
1,000 posts - upgrade of space limit