Thanks Ryan Vincent and simON for your help! :) I've finally (After many hours and many headaches) found the solution, as you can see bellow, i find the mine more simple:
<?php
if(empty($_GET['i'])){
//displays a error image if GET['i'] is empty
$im = imagecreatefrompng("scr/m/e.png");
//keeps the transparency of the picture
imagealphablending( $im, false );
imagesavealpha( $im, true );
// Output and free memory
header('Content-Type: image/png');
imagepng($im);
imagedestroy($im);
}elseif(!file_exists('img/'.$_GET['i'])){
//displays an error image if the file doesn't exists in the img folder
$im = imagecreatefrompng("scr/m/e.png");
//keeps the transparency of the picture
imagealphablending( $im, false );
imagesavealpha( $im, true );
// Output and free memory
header('Content-Type: image/png');
imagepng($im);
imagedestroy($im);
}else{
// Load the stamp and the photo to apply the watermark to
$stamp = imagecreatefrompng('scr/m/w.png');
$get = $_GET['i'];
$im = imagecreatefromjpeg('img/'.$get);
// get the height/width of the stamp image
$sx = imagesx($stamp);
$sy = imagesy($stamp);
$sximg = imagesx($im);
//percentage of the size(5%)
$percent = $sximg * 0.15;
//positionnig the stamp to the center
$posx = round(imagesx($im) / 2) - round($percent / 2);
$posy = round(imagesy($im) / 2) - round($percent / 2);
//Create the final resized watermark stamp
$dest_image = imagecreatetruecolor($percent, $percent);
//keeps the transparency of the picture
imagealphablending( $dest_image, false );
imagesavealpha( $dest_image, true );
//resizes the stamp
imagecopyresampled($dest_image, $stamp, 0, 0, 0, 0, $percent, $percent, $sx, $sy);
// Copy the resized stamp image onto the photo
imagecopy($im, $dest_image, round($posx), round($posy), 0, 0, $percent, $percent);
// Output and free memory
header('Content-type: image/jpg');
imagepng($im);
imagedestroy($im);
}
?>
Thank you again for your replies. I hope my script will help everybody in the same situation that I was! :) Good night (I'm french, it's the night :3 ) !