Open a 'Photobucket' account (it's free);
Upload your photo's from your PC to your account. When they are saved type your reply to any thread you want to add a photo to, go back to Photobucket, put your mouse over the photo you want and you'll get a drop-down menu, click on the box next to 'IMG Code' and it will say 'copied.'
Come back to your reply, right click and 'paste' and your photo will be available for all to see...