Going in order of implied questions and explicit questions:
There’s a “meta” section for comments, queries and suggestions about the Discourse forum. You can probably move the thread there yourself if you want, or ask a mod to do so… I’m not sure if you’ll have the move option as a new user though. Either way, it’s useful to acquaint yourself with the different forum areas – there’s heaps of cool content tucked away in unexpected corners of the Discourse 
As for question i), your limits will naturally grow/increase as you take part in activities within the community. Newer users are, unfortunately, capped lower purely to prevent spam. Fortunately though, you’ll rank up pretty quickly if you’re active; and the moderators can extend permissions if you need to post many images or something like that.
(Some friendly advice, on the assumption that your question is regarding your photo-journal idea – if I may be so bold, I’d suggest you limit your posts to one major update on that thread every day, and save up material to get a good long streak going. That way you’ll have plenty of other posts to take part in other discussions, and it will help you create good discipline to post powerful, narrative images so your story keeps a cracking pace. I’ve tried… oh, it must be over a dozen times now to post a photo journal of a town-in-progress, and every time I run into the issue of having hundreds of screenshots with very little town progress to show for it. I end up either getting bored with the story, or being overtaken by a new alpha and starting the story of a new town to take advantage of that alpha’s new features. If you think within the mindset of having a limited number of pictures to show your story unfolding, it will prompt you to be more creative with your words and to make your pictures more powerful as a result; and hopefully allow you to crank out material more quickly as well 
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The answer question ii) was kind of included above: either rank up, or ask a mod for help. You can search for the “moderators” user group and page one of them, or send them a private message explaining the situation.
For question iii), it’s basically a way to keep track of activity within the community. Long-term members who are active a lot will naturally attain a high rank with little effort, so it helps the developers to tell who among the community is contributing regularly. Back when the community was smaller, it was a great way to help pick out the leaders who were shaping the community and to recognise their efforts. Now, we have hundreds of leaders and tons of awesome content; so the system doesn’t have as much of a visible impact. However, it’s still a pretty good way to keep track of your own activity. I’m the first to admit that once I saw the “goals”/achievements, I became more excited to keep taking part and clock up those shiny milestones hahah!
In short, if you take part and have fun you’ll find you quickly fit into the discussions and you’ll be able to participate more and more; the background systems will step back and let you take your place as an “experienced” member of the community. It seems a bit draconian and complex to impose these limits on newbies just for being new; but then, the alternative is something like the Steam forums where there’s nothing to foster discussion other than posters’ good will. That can work still work out beautifully of course, but it doesn’t always; Discourse’s systems are designed to gently promote a sort of open and active dialogue (i.e. talking to each other) rather than people talking at each other.
So, welcome to the Discourse! You’ve just endured one of my (in)famous walls of text, but if you have any other questions I’ll try to give more specific answers… assuming that the other forumgoers don’t beat me to it heheh. You’ll find we’re a friendly bunch here, and I look forward to seeing you around
