How can I alter (theme) the default content feed? How can I alter (theme) the default content feed on the front page (the list of node teasers) so that my 'thumbnail' image style is used instead of my 'medium' style for the images contained in the nodes.

All I can think of is to create my own page--front.tpl.php file or use is_front in node.tpl.php. Either way it means creating a custom listing for the front page.

Thanks.
The best way to do this if you're running stock D7 is in config: you'll want to change the default field dislpay settings by clicking "manage display" on the Article content type in admin/structure/type. 

Click the gear icon to adjust the display type: