It should be in a notice at the top of the page for new members. It says something like this:
Hi!
Welcome to TMC! Since you currently have few or no posts in the forum, you have somewhat limited profile privileges. You'll be able to do things like add a profile image after you've participated in the community a bit more. Please review the Forum Rules and happy posting!
The notice is just a notice of limited privileges. It would be more helpful if there was a link explaining how many posts people need before their privileges are available.
Very informative, just joined after lurking for a long time, and I was looking forward to adding an avatar pic but need more posts...and my car to actually arrive!