$picture = ""; # picture fileNAME here. not address
$max=150;    # maximum size of 1 side of the picture.
/*
here you can insert any specific "if-else",
or "switch" type of detector of what type of picture this is.
in this example i'll use standard JPG
*/
$src_img=ImagecreateFromJpeg($picture);
$oh = imagesy($src_img);  # original height
$ow = imagesx($src_img);  # original width
$new_h = $oh;
$new_w = $ow;
if($oh > $max || $ow > $max){
    $r = $oh/$ow;
    $new_h = ($oh > $ow) ? $max : $max*$r;
    $new_w = $new_h/$r;
}
// note TrueColor does 256 and not.. 8
$dst_img = ImageCreateTrueColor($new_w,$new_h);
ImageCopyResized($dst_img, $src_img, 0,0,0,0, $new_w, $new_h, ImageSX($src_img), ImageSY($src_img));
ImageJpeg($dst_img, "th_$picture");