Hey Cinnamonteal - I'd give you all the steps, but it's been done better here. If you think you've got it figured out and want to give it a test shot, try posting an image in the help forum first.
THe short of it is this:
If you want to post your own pictures, you need some place to store them (most people use photobucket here, so get yourself an account.)
Then when it comes time to post an image, you can post the URL or sites like photobucket generally put all the correct order in for your and you can just copy and paste) I'll skip the long way, just look here: