A good place to start is with the template hierarchy flow chart. http://codex.wordpress.org/images/1/18/Template_Hierarchy.png I’m always consulting this page.
When you say modified twenty eleven, what do you mean? The image.php file is what handles image attachment pages in twenty eleven. I’m not familiar entirely with how it works, but it may be also making a call to the content-image.php file where the_content() function is being invoked, maybe. Not sure.
Hopefully this’ll at least get you pointed in the right direction though.