Ok I found an other way to do this:
$count = str_word_count($string);
$split = explode(' ',$string);
$part1 = array_slice($split,0,ceil($count/2));
$part2 = array_slice($split,ceil($count/2));
$string1 = implode(' ',$part1);
$string2 = implode(' ',$part2);
first I count the words of the database, then I splitted it of after the half, and put the 2nd picture at the top oft the second half.
Thats not pefect I think, but I´m contented :)