questa mancava, se ci sono altri errori non so'Codice PHP:
<?
$imgSource="/var/www/htdocs/immobiliare/img/big/default.jpg";
$imgSource = imagecreatefromjpeg($imgSource); // <------
$srcX=0;
$srcY=0;
$srcW=200;
$srcH=200;
$destX=0;
$destY=0;
$dstH=75;
$dstW=100;
imagecopyresized($imgDest, $imgSource, $destX, $destY, $srcX, $srcY, $dstW, $dstH, $srcW, $srcH);
header("Content-type: image/jpg");
imagejpeg($imgDest);
?>