Actually those images are thumbnails, but they are automatically resized to whatever the size of the div that they are held in. If you take a look at your home.php file you’ll notice that they are created with the class – “Featured”. So in your functions.php file you can add an image size that will change anything labeled “Featured”….which should be just on the homepage. You can add the image size code in your functions.php file right under the other two (near the top) –
add_image_size( 'Featured', 220, 150, TRUE );
You’ll alos want to download and run a plugin called Regenerate Thumbnails – http://wordpress.org/extend/plugins/regenerate-thumbnails/ ….if you haven’t already. This will automatically adjust all of your thumbnail images to the correct size.