customising the “Insert Into Post” code

Once i’ve uploaded an image using the flash uploader i’d like the code which is sent to the post when clicking “Insert Into Post” to be a little different. Currently the code will look something like this:

<img src="https://www.server.com/wp-content/uploads/2008/11/campbell-66-copy.jpg" alt="" title="campbell-66-copy" width="500" height="250" class="alignnone size-full wp-image-37" />

Ideally i’d like the class to automatically be set as a style i’ve defined, e.g class="thumbnail" i’ve had a look around the wp-admin files but can’t isolate where this information is stored.

Thanks in advance
m