Add a code check to stop these from being changed:

//Check and Set the Default Thumbnail Sizes
if(get_option('thumbnail_size_w')!=160)update_option('thumbnail_size_w',160);
if(get_option('thumbnail_size_h')!=80)update_option('thumbnail_size_h',80);
if(get_option('medium_size_w')!=640)update_option('medium_size_w',640);
if(get_option('medium_size_h')!=320)update_option('medium_size_h',320);
if(get_option('large_size_w')!=1280)update_option('large_size_w',1280);
if(get_option('large_size_h')!=640)update_option('large_size_h',640);

Then they can be called with get_the_post_thumbnail:

$id=$post->ID;
get_the_post_thumbnail($id, 'thumbnail'); // Thumbnail
get_the_post_thumbnail($id, 'medium'); // Medium resolution
get_the_post_thumbnail($id, 'large'); // Large resolution
get_the_post_thumbnail($id, array(480,240) ); // Other resolutions

References:
WordPress.org Forums