The easiest way to create an image link is to use a shortcode from the oik plugin.
You can create a text link using the [bw_code bw_link] shortcode.
e.g. [bw_link 402]
Similarly you can create an image link using the [bw_code bw_thumbs] shortcode
e.g. [bw_thumbs id=402]
The image that is displayed is the featured image for the post, or if there is no featured image it’s randomly selected from the images attached to the post.