You can either upload directly from your computer, but the pic has to be pretty small in size to do it that way. The button to do that is on the page when you are posting, just below where you type the body of the message ( it says 'attach file' ).
The other way, and the easier one in my opinion is to upload to a web host that supports remote viewing, then you just link here using the [IMG] tags. The easiest way to do this would be to click the IMG button above where you are typing right now and it allows you to attach the url for the image.
I know it all sounds confusing, just give it a try to practice. Maybe someone else can chime in here and put this in better English.