add_action( 'genesis_before_post', 'agency_post_image' );
function agency_post_image() {

if ( is_category() ) return;
if ( $image = genesis_get_image( 'format=url&size=post-image' ) ) {
printf( '<a href="%s" rel="bookmark"><img class="post-photo" src="%s" alt="%s" /></a>', get_permalink(), $image, the_title_attribute( 'echo=0' ) );
}
}