yingkhun on "help me about thumbnail"

index.php

$thumbID = get_the_post_thumbnail( $post->ID, 'background', $imgsrcparam ); ?>
<div class="preview"><a><?php echo "$thumbID"; ?></a></div>

function.php

add_theme_support( 'post-thumbnails' );
set_post_thumbnail_size( 305, 9999 );
add_image_size( 'background', 305, 9999 );

i need to use original image(url) and resize to 305,xxx
but this code make thumbnail size and low quality.

ex.
original url :
http://www.test.com/img.jpg

this code make :
http://www.test.com/img-305x252.jpg

i want to use original url and resize to 305x252 (it's make high img quality)
how to edit this code

ps.sorry about bad in english and thanks