Theming image_attach

I do see all of the image sizes on the image node page, but what I'm trying to do is get a different image size to display on the story page (to which the image is attached). I've run into this same problem a lot. I want to customize the output of a node in the node-[whatever].tpl.php file, but the structure of the node object is a complete mystery to me. I'm not terribly familiar with PHP, but I know enough to write an "if" block or a "foreach" block. The problem is, I don't understand how to separate out the different parts of the node. In the past, I've used the contemplate module, which usually does an excellent job of revealing all those data structures that make up the node so I've figured out many times how to do what I want to do.
The problem now is that I'm only aware of one variable within the node structure that allows me to access the attached image: $node->content['image_attach']
So, this variable is apparently an array, but I'm only aware of the [#value] part of that array, which outputs the thumbnail image size within a series of predefined HTML that I could probably do without. What I'm trying to do is figure out how to get [#value] to output a different size of the image.