Simple instructions.

1. 'Copy' the url from the upload page.
2. Insert it here, within your regular message, between the words 'pic' and '/pic' both between square brackets.
That dont sound simple, does it?

Look at this example but forget I put spaces in there. (if I take out the spaces it will try and interpret it as a picture)
When you type yours, dont put any spaces in.

[p ic]yourfilename.jpg[/p ic]