function changeImage(filename, width, height)
{
    if (width == undefined)
        width = 512;

    if (height == undefined)
        height = 384;

    document.mainimage.style.visibility='hidden';
    document.mainimage.width = width;
    document.mainimage.height = height;
    document.mainimage.src = filename;
    document.mainimage.style.visibility='visible';
}