Photos are easy once you know how but it's amazing how many people have problems at first.
First you need to have the picture hosted somewhere. If it's a picture you've taken then something like Flickr is the best option.
Then once the picture is loaded up (or if it's an image that already sits on another website), just right click on your mouse and select 'Copy Image Location'.
All you need to do is paste that link into the text bar that is generated when you click on the 'IMG' button on the dialogue box when you're posting to the forum.
Hope that helps for future reference.