why does the_post_thumbnail not return a thumbnail (80×80) ?

All,

I coded this

if ( has_post_thumbnail() ) { ?>
” rel=”bookmark” title=”<?php the_title(); ?>”>
<?php the_post_thumbnail(‘thumbnail’,array(‘class’ => ‘left’,’alt’ => ”.get_the_title().”,’title’ => ”.get_the_title().”)); ?>

But the image I get back from the_post_thumbnail is not allways the 80×80 image for a post ? It seems to be just the image that is set as ‘default’ image.

For newer posts I use the auto_post_thumbnail to make sure the ‘default image’ is set to the thumbnail (80×80) image.

Is there a way to modify my older posts ?
Is the above code wrong and should I do something else to get the 80×80 image ?
In trying to find this I looked into the get_the_post_thumbnail function , but I get lost at the line
$size = apply_filters( ‘post_thumbnail_size’, $size );
line… how does this line work ?