Pokud jsi na to jeste neprisel, tak tady to mas komplet:
$in_name = "../images/obrazek.jpg";
$out_name = "../images/obrazek_small.jpg";
$in_size = getimagesize($in_name);
$out_size[0] = ceil($in_size[0] / ($in_size[1] / 112)); // dopocte druhy rozmer,aby byl nahled maximalne 112px vysoky - sirka pak vychazi 150px
$out_size[1] = 112;
$in = ImageCreateFromJpeg($in_name);
$out = ImageCreateTrueColor($out_size[0],$out_size[1]);
ImageCopyResampled($out,$in,0,0,0,0,$out_size[0],$out_size[1],$in_size[0],$in_size[1]); // Resampled, pokud mas na serveru knihovnu GD2. Kdyz nemas, tak prepis Resampled -> Resized
ImageJpeg($out,$out_name,75);
ImageDestroy($in);
ImageDestroy($out);
Jinak na to existuje milion clanku...