PHP imagecopy功能根据主图像的高度和宽度改变图章大小

I'm trying to add a watermark (logo) to each image upload to the website.

So, I used imagecopy PHP function to add a watermark (a png image) to the main image (a jpg image) but the problem is the logo size is changing according to the main image size (height and width), That's mean if I upload a 4000x2000 image the logo with be somthing like 100x100 and if the main image size is 1000x500 the stamp will be bigger than the real size (546x537).

Image Samples:

https://crkemlak.com/appimg/199f8486d7d77007771f2f450dffca4d.jpeg
https://crkemlak.com/appimg/d6f9fd02999eced76eac9a6995df904f.jpeg
https://crkemlak.com/img/stamp.png

I used this code to add the watermark to the image:

$im = imagecreatefromjpeg('../appimg/'.$filenamerand);

$originalWidth= imagesx($im);
$originalHeight = imagesy($im);

$stamp = imagecreatefrompng('../img/stamp.png');
$marge_right = 10;
$marge_bottom = 10;
$sx = imagesx($stamp);
$sy = imagesy($stamp);
imagecopy($im, $stamp, ($originalWidth-$sx)/2, ($originalHeight-$sy)/2, 0, 0, imagesx($stamp), imagesy($stamp));

I need your help please to fix this problem, I need to make the watermark is in it's real size in any size of the main jpg image

Thanks