You have an error in your SQL syntax; check the manual....

<?php
session_start();
$id = $_SESSION['id'];
include("scripts/connect_to_mysql.php");
$fotka_left = $_POST['fotka_left'];
$fotka_top = $_POST['fotka_top'];
$info_left = $_POST['info_left'];
$info_top = $_POST['info_top'];
$jmeno_left = $_POST['jmeno_left'];
$jmeno_top = $_POST['jmeno_top'];
$string = "px";
 $a = preg_replace("/[^0-9]/","314", $fotka_left);
 $b = preg_replace("/[^0-9]/","109", $fotka_top); 
 $c = preg_replace("/[^0-9]/","316", $info_left);
 $d = preg_replace("/[^0-9]/","288", $info_top);
 $e = preg_replace("/[^0-9]/","680", $jmeno_left);
 $f = preg_replace("/[^0-9]/","73", $jmeno_top);

 $a = $fotka_left.$string;
 $b = $fotka_top.$string;
 $c = $info_left.$string;
 $d = $info_top.$string;
 $e = $jmeno_left.$string;
 $f = $jmeno_top.$string;
 


$q = mysql_query("UPDATE uzivatele SET fotka_left='$a', fotka_top='$b', info_left='$c', info_top='$d', jmeno_left='$e', jmeno_top='$f' WHERE id='$id'");
$result=mysql_query($q);
if($result){
echo " Hotovo, <a href='http://test-income.mzf.cz/profil.php?id='$id''>Zpět na profil</a>";
}
else {
echo mysql_error();
echo "<br>";
echo "Chyba, <a href='http://test-income.mzf.cz/profil.php?id='$id''>Zpět na profil</a>";
echo "<br>";
echo $id;
echo "<br>";
echo $a;
echo "<br>";
}
mysql_close();
?>