When you start a new thread or click on the "reply to topic" link at the top right of an existing thread you get the full "compose message" window. Just below the text entry area you will see a little paper-clip. Click on that icon, click on the browse button and select your photo that is smaller than 250KB big, and select "upload". That's it.
Now, if it is something you want in your photo gallery too, that is a different process.
GK