first image of a wordpress post.

 <?php if (have_posts()) : ?>
 <?php while (have_posts()) : the_post(); ?>

 <?php
 $szPostContent = $post->post_content;
 $szSearchPattern = '~<img [^\>]*\ />~';

// Run preg_match_all to grab all the images and save the results in $aPics
preg_match_all( $szSearchPattern, $szPostContent, $aPics );

 // Check to see if we have at least 1 image
 $iNumberOfPics = count($aPics[0]);

if ( $iNumberOfPics > 0 ) {
   // Now here you would do whatever you need to do with the images
   // For this example the images are just displayed
   for ( $i=0; $i < $iNumberOfPics ; $i++ ) {
        echo $aPics[0][$i];
   };
};

endwhile;
endif;
?>