I'm assuming you're going to have to use some javascript to create this effect. PHP takes all of the code you send it and generates an html page that it sends to the browser.
Maybe something like:
<script language=javascript>
function waitasec()
{
setTimeout("displayimage('image.jpg')",1250);
setTimeout("displayimage('image2.jpg')",1250);
setTimeout("displayimage('image3.jpg')",1250);
}
function displayimage(image){
document.write('<IMG SRC="' +image+ '">');
}
</script>
<body onload="waitasec();">
Although I believe document.write erases everything everytime it runs, so you will probably have to have a hidden div and have the timeout script change its visibility to true.