If you want to post a picture in a post or your signature, you will have to copy the image's URL code and paste it where you want it. If you find a picture that already has a given URL code, such as an image from the internet, use this code...
[img]URL HERE[/img]
However, if you are using a picture from your computer, you will have to upload it to someplace like TinyPic or Photobucket, and from there, copy the URL code.
If you want to upload a picture as your forum avatar, there is an option for you to do that if you go to the "Profile" tab.