It seems that you are nearly there. All you have to do is retrieve the passed variables $Pet and $Colour from the $HTTP_POST_VARS array and check them before you print them as an image.
If you look at the ubbt.inc.php the getinput function is a very good start on how to retrieve variables passed on your script. All you have to do then is check if those variables are empty and if not then you will either display a default image or nothing at all
if (!$Pet and !$Colour) {
// display nothing
} else {
echo "<img src="$Pet" />$Colour";
}
I haven't tried this but it should work.
