<?php
// retreives image from the post
function getImage($num) {
global $more;
$more = 1;
$content = get_the_content();
$count = substr_count($content, '<img');
$start = 0;
for($i=1;$i<=$count;$i++) {
$imgBeg = strpos($content, '<img', $start);
$post = substr($content, $imgBeg);
$imgEnd = strpos($post, '>');
$postOutput = substr($post, 0, $imgEnd+1);
$image[$i] = $postOutput;
$start=$imgEnd+1;  
 
$cleanF = strpos($image[$num],'src="')+5;
$cleanB = strpos($image[$num],'"',$cleanF)-$cleanF;
$imgThumb = substr($image[$num],$cleanF,$cleanB);
 
}
if(stristr($image[$num],'<img')) { echo $imgThumb; }
$more = 0;
}
//retreive image ends
?>

<!--thumnailcode-->
<div class="thumbnail">
<a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>">
<img src="<?php bloginfo('template_directory'); ?>/timthumb.php?src=<?php getImage('1'); ?>&w=150&h=150&zc=1">
</a>
</div>

<!--css-->
.thumbnail a:link, .thumbnail a:visited {display:block; float:left; padding:5px; background:#e2e2e2; width:150px; height:150px; margin:5px 5px 0 0;}
.thumbnail a:hover, .thumbnail a:active {background:#C4C4C4;}

http://www.problogdesign.com/wordpress/automatic-wordpress-thumbnail-without-custom-field/

This post will show you how to make your theme generate thumbnails automatically, based on your post’s first image. Uses timthumb to resize

AUTOMATIC WORDPRESS THUMBNAIL WITHOUT CUSTOM FIELD

wordpress

PHP