How To Set Default Attachment Display Settings In WordPress

This is a pretty quick fix.

Add this to your functions.php for your theme and then you can set the default for the image attachment settings

function mytheme_setup() {
 // Set default values for the upload media box
 update_option('image_default_align', 'center' );
 update_option('image_default_link_type', 'media' );
 update_option('image_default_size', 'large' );

}
add_action('after_setup_theme', 'mytheme_setup');

Obviously changing the values to what you want as the default.