Generates a random image from your list. Simply add more array values with your image url.

GD Functions must be enabled in the PHP setup.

Code:
<?php

// Images array
$imgs = array();
// add $imgs[__current#__] = "__IMGAGE_URL__"; for more images
// image URLs must be relitive to the script
$imgs[0] = "sig_one.jpg";
$imgs[1] = "another_sig.jpg";
$imgs[3] = "new_sig.gif";
$imgs[4] = "yet_another_sig.png";

// choose a random image
$num = rand(0, sizeof($imgs) - 1);
// do a check
if(is_file($imgs[$num])){
	// get type
	if(strstr($imgs[$num], ".gif")){
 *// display gif image
 *header("Content-type: image/gif");
 *$im = imagecreatefromgif($imgs[$num]);
 *imagegif($im);
	}elseif(strstr($imgs[$num], ".jpg") || strstr($imgs[$num], ".jpeg")){
 *// display jpg/jpeg image
 *header("Content-type: image/jpeg");
 *$im = imagecreatefromjpeg($imgs[$num]);
 *imagejpeg($im);
	}elseif(strstr($imgs[$num], ".png")){
 *//display png image
 *header("Content-type: image/png");
 *$im = imagecreatefrompng($imgs[$num]);
 *imagepng($im);
	}else{
 *die("ERROR2!!!");
	}
}else{
	die("ERROR!!!");
}

?>
Simply call the script as an image.

Example:
http://wade.realestateleavenworth.com/wadesig.php

Refresh it and it might change.

Note: Animated gifs lose there animation and it stays on the first frame.
Note2: IPB doesn't allow images with .php extentions so you can't you it for Invisionfree boards. Not sure about phpBB2 yet.