changing rendered html

Hello,

Im building my own theme using a copy of the default kubrick theme and i need some pointers on which files to change so i have a different output of html. Currently when i attach an image in making a post, the theme outputs the following:
<p><a href="#"><img width="65" height="85" alt="" src="#" title="books [left aligned]" class="alignleft size-medium wp-image-5"/></a></p>

How can i change the code where the image isn’t wrapped in a p but something else like a div.