Open a reply, look below the white reply box. There in blue is 'additional options', click on that, a drop down list appears, click on 'choose', this takes you to your own computer files, choose your photo and click 'open', then click 'post'. As Sandy says the file cannot be more than 512 kb so you need to make it smaller if your photo is quiet big - many are more or less that size anyway though. If you save the smaller version onto your desktop with a different name it will be easier to find and you can keep your original file the same size.
Dan - any chance you could have a 'how to' section on the new site?