Here is an example to make black square on a white background (black.php)
<?php
$image = imagecreate(200, 200);
$white = imagecolorallocate($image, 0xFF, 0xFF, 0xFF);
$black = imagecolorallocate($image, 0x00, 0x00, 0x00);
imagefilledrectangle($image, 50, 50, 150, 150, $black);
header("Content-Type: image/png");
imagepng($image);
?>
Now use as an image
<img src="black.php" />