$types = array( 'png', 'jpg', 'jpeg', 'gif' );

if ( $handle = opendir('pasta_desejada') )
{
    while ( $entry = readdir( $handle ) )
    {
        $ext = strtolower( pathinfo( $entry, PATHINFO_EXTENSION) );

        if( in_array( $ext, $types ) ) {
              echo '<img src="pasta-relativa-por-http/', $entry,'">';
        }
    }

    closedir($handle);
}