Toms method I think is is using a custom field and mine is using the built in post (“feature”) image.
If I am reading this right you want the first post in the loop to have a larger image that the rest.
The different divs are not really required just use a variable inside the ‘loop’ which would work for both tom’s and my methods.
As you are using twenty ten which uses WordPress post thumbnail support, this would be my solution using the WordPress built in feature where post image support and the default size has already been set.
I would add the ‘first post’ and ‘list-post’ (standard image ratio: 4:3) image sizes in functions.php
if ( function_exists( 'add_image_size' ) ) {
add_image_size( 'first-post', 200, 150 );
add_image_size( 'list-post', 100, 75 );
}
Then in the loop.php (template part) find the content divs (there are two that you may want to update) and inside add the thumbnail support
<?php if(!$fisrtimage) : ?>
<?php $fisrtimage=true; ?>
<?php if ( has_post_thumbnail()) the_post_thumbnail('first-post'); ?>
<?php else: ?>
<?php if ( has_post_thumbnail()) the_post_thumbnail('list-post'); ?>
<?php endif ?>
David