There was a bit of stuff in the FAQ, but admittedly it wasn't that clear so have added this:
Q: Erm... OK, so how do I do that? (become a content author)
A: Firstly signal your intent to publish some stuff. This is done on your profile. Go there, under the 'Edit' and 'Content' sub-tab you'll find a little box to check if you want to become a content author. In a short time (usually less than an hour) you'll be granted rights to post pictures in the galleries and ramble on to your heart's content
Q: So I'm a content author, but I still don't get it. Help!
A: Ooops, sorry, not the most intuitive thing, eh? On the left you should see a little box with your username atop and a 'create content' link. This may or may not be expanded, but when it is there should be options to add various stuff (forum topic, image (pictures), etc.) - is this helping?
There was a bit of stuff in the FAQ, but admittedly it wasn't that clear so have added this: