var img=new Image();
img.src="/1.gif"; //картинка скачалась и попала в кэш
...
document.images[ 0 ].src=img.src; //на месте первой картинке на странице появилась  1 .gif

function oCreateRQ() {
    http_request = false; 
    if (window.XMLHttpRequest) { // Mozilla, Safari,... 
        http_request = new XMLHttpRequest(); 
        if (http_request.overrideMimeType) { 
            http_request.overrideMimeType('text/xml'); 
        } 
    } else if (window.ActiveXObject) { // IE 
        try { 
            http_request = new ActiveXObject("Msxml2.XMLHTTP"); 
        } catch (e) { 
            try { 
                http_request = new ActiveXObject("Microsoft.XMLHTTP"); 
            } catch (e) {} 
        } 
    } 
    if (!http_request) { 
        alert('Giving up :( Cannot create an XMLHTTP instance'); 
        return false; 
    } 
    http_request.onreadystatechange = alertContents; 
    return http_request;
}

function SendRQ(sRQ) {
    http_request = oCreateRQ(); 
    http_request.open('GET', sRQ, true); 
    http_request.send('5'); 
}

function makeRequest(idgoods) {
    var RQ = '/client/gdesc?id=' + idgoods;
    SendRQ(RQ);
}

function alertContents() {
    if (http_request.readyState ==  4 ) { // COMPLETED
        if (http_request.status ==  200 ) { // HTTP STATUS
            var response = http_request.responseText;
            var update = new Array();
            if(response.indexOf('|' != - 1 )) {
                update = response.split('|');
                document.getElementById(update[ 0 ]).innerHTML = update[ 1 ];
            } 
        } else { 
            alert('There was a problem with the request.'); 
        }
    }
}

    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        String ID = request.getParameter("g");
        String S = "wrs" + ID + "|<img src=pict" + ID  + ".jpg>";
        out.println(S);
        out.close();
    }