Here is a quick and very easy way to select and display a random image each time the page is loaded. Store the image links in an array and with the use of array_rand  a random one will be pulled from the array and echo’d in the img src:

$images = ["cat.png","dog.png","mouse.png","bird.png"];
$random_image = $images[array_rand($images)];

echo "<img src='$random_image'>";

I told you it was easy.