PHP删除以前的循环数据

Javascript won't run the "++" operator once per second - it will run it twenty times, very fast. You should use the setInterval function to get the js to only call every 20 seconds:

http://www.w3schools.com/jsref/met_win_setinterval.asp

Change your JS (Untested!!!):

$(window).load(function(){
    setInterval(function(){
        //create XMLHttpRequest object
        xmlHttpRequest = (window.XMLHttpRequest) ?
            new XMLHttpRequest() : new ActiveXObject("Msxml2.XMLHTTP");

        //If the browser doesn't support Ajax, exit now
        if (xmlHttpRequest == null)
            return;

        //Initiate the XMLHttpRequest object
        xmlHttpRequest.open("GET", "../php/rotalas.php", true);

        //Setup the callback function
        xmlHttpRequest.onreadystatechange = function(){
            if(xmlHttpRequest.readyState == 4){
                document.getElementById('friss_kepek').innerHTML = xmlHttpRequest.responseText;
            }
        };

        //Send the Ajax request to the server with the GET data
        xmlHttpRequest.send(null);
    }, 20000); //Run every 20000ms
}

Edit: and your PHP should be something like this:

$imgdir = '../img/blog/img/amator/Amator_thumb/'; 

$i=0;

$dimg = opendir($imgdir);//Open directory
while($imgfile = readdir($dimg))
{
    if( in_array(strtolower(substr($imgfile,-3)),$allowed_types) OR
        in_array(strtolower(substr($imgfile,-4)),$allowed_types) )
        /*If the file is an image add it to the array*/
    {$a_img[] = $imgfile;}
    if ($imgfile != "." && $imgfile!="..")
    {
        $imgarray[$i]=$imgfile;
        $i++;
    }
}
closedir($imgdir);

$totimg = count($a_img);  //The total count of all the images .. img_coun

for($x=$page*1; $x < $totimg && $x < ($page+1)*1; $x++){
    $rand=rand(0,count($imgarray)-1);
    if($rand >= 0)
    {
        echo '<img class="kep_listaz" src="../img/blog/img/amator/Amator_thumb/'.$imgarray[$rand].'" width="160" height="140">';
    }
}

I haven't tested this at all, but the general idea should be there. The javascript uses setInterval to make an AJAX call every 20s, then the PHP instantly responds with some images. It's up to you to figure out how to get just the images you want.

The thing to take away from this is: Don't use for loops for timing! That's something called CPU limiting which is something that shouldn't be done unless you really really know what you're doing.

Good luck!

As Erty mentioned, your code runs 20 times very fast, so it basically sends 20 requests at once. If you can set recursive function with timeout of 20 seconds.

Ex:

function updatePic(counter){
  setTimeout(function(){
    //do ajax call and you can use counter to determine what picture to return
    updatePic(counter++);
  }, 20000);
}

You don't need any timer related stuff for PHP side. Note that you can run an ajax call outside or inside of setTimeout() - depending what you want to do on the first loop (call the function then wait 20 seconds before making an ajax call, or make an ajax call every 20 seconds, the first one happening instantly when you call the function).

The better option is to use slideshow plugins for javascript/jquery. There are plenty of options, and my favorite is cycle.