If it's an image already in your personal album, there's two types of links available below it when you view it - just copy either of the links into your post and it will come up. One just displays the standard image, and one makes a clickable image to the larger version.
If it's not in your person album (as in, hosted elsewhere), just post the link to wherever it is inside the 'Img' tags shown in the toolbar above the posting input box.