Firefox no properties for document.images?

[Palko17]

I merged my earlier posted script with another script, the AJAX works now, but it won't change the images to the correct src... I have no idea what JS's problem is, but it's got serious ones...

var offsetx=0;
var offsety=0;

function getKey(e, thisoffsetx, thisoffsety, firstx, firsty) {

    var firstx=offsetx+firstx;
    var firsty=offsety+firsty;

    if (e.keyCode=='38') {

        for (var y=1; y<=17; y++) {

            thisfirstx=firstx;

            for (var x=1; x<=20; x++) {
                thisfirstx++;

                if (window.XMLHttpRequest) { 
                    myRequest = new XMLHttpRequest();
                } else if (window.ActiveXObject) {
                    myRequest = new ActiveXObject("Microsoft.XMLHTTP");
                }

                myRequest.onreadystatechange = function() {

                    if (myRequest.readyState == 4) {

                        var thisimageid="mapTile"+x+"_"+y;

                        var imageLink="*********"+myRequest.responseText;

                        document.thisimageid.src=imageLink;

                    }

                }

                myRequest.open('GET', '************?x=' + thisfirstx + '&y=' + firsty + '&direction=up', true);                  
                myRequest.send(null);

            }

            firsty++;

        }

    offsety= thisoffsety-1;

    }

}