Posting limits and user abilities are all based on trust. As a user engages with the community, the user grows trust and is rewarded with more abilities. Trust is not just based on the number of posts of a user, but the number of topics read and the time actually spent browsing the community.
When a user signs up, the user begins with no trust. This is to prevent abuse of new accounts and reduce spam as the community grows.
After posting a few topics, reading some stuff, and being a part of the community, the user will be given basic trust and is allowed to upload images and invite users to the community. The more trust a user gains, the more offsite links and images can be posted.
When a user is invited to the community by an existing user, they begin with basic trust, removing the need to "prove themselves." So, if you know people who would be a great part of the community, invite them!
Any question you have regarding the restriction system, feel free to ask!