There is a bright RED button in every forum section that you are allowed to post in that says POST NEW THREAD. Same way you posted this one. It doesn't change per forum section.
As far as posting pictures, there is a thread in the off topic section that walks you through how to do it. If you are asking where, you can make a thread in the members ride section, or the build section, or the photography section...depends on what you are trying to accomplish.
I recommend LOOKING around the forum before diving in. The sections are broken up into their own respective areas and there are certain sections for everything for a reason. It not only helps us moderators keep this place clean, but it also helps the members like you to be able to find stuff easier. You are making this out to be way harder than it has to be
It is actually pretty easy and laid out very cleanly. Justin spent a lot of time building this into the format that it is now and it works really well compared to other forums that are a cluttered mess.