draw_rectangle.php

<?php
$url = "http://31.media.tumblr.com/tumblr_mc4u6lwZHr1qf4k86o1_500.jpg";
$img_string = file_get_contents($url);
$img_data   = imagecreatefromstring($img_string);

// make color
$gd  = imagecreatetruecolor(1,1);
$red = imagecolorallocate($gd, 255, 0, 0);

// drawing
for ($x = 1;$x<100; $x++) {
    for ($y=1;$y<100;$y++) {
        imagesetpixel($img_data, $x, $y, $red);
    }
}
// output
imagepng($img_data, '/tmp/test.png');