Just generally speaking, it is not a "fourm" (I think you meant forum, which encompasses the entirety of the site), I believe what you're referring to is creating a topic or thread on the Canterlot Castle subsection, if that's the case, here's your answer.
For the answer to your first question, you can't make new topics in the Canterlot Castle subsection, as it is used by the site staff to make announcements. For certain threads there, you can post replies, which works just like posting any reply here on the forums.
For your second question, by making posts anywhere but Cloudsdale Colosseum. To get to the next rank (Muffin), you need three more as of the time as this posting, as you already have two active posts. Make sure you're making quality posts, as making low-quality posts just to rank up and not actually contribute would be considered spam, which is against our global rules here on MLP Forums.
If you have any more questions, I'd be glad to assist.
Happy foruming, I'll see you around.
Edit:
Also, I'd recommend not asking for bro hooves, if people like your post, they will bro hoof it, they don't need to be told to. Another thing I'd recommend is not signing your posts, instead, I'd recommend using a signature (Which automatically appears below all of your posts) or just not doing it as your username appears directly to the left of all your posts.
You can't upload it directly from your files, just upload the picture to imgur.com or any image hosting site and then copy the image url, that is the fastest way to do it.
No, it doesn't change anything, really. The fact that you can only use images hosted on the web is something that is part of the website itself.
Harmonic Revelations's post in How can i make a fourm on the castle? was marked as the answer