function abrir_imagem(df,imgx,imgy){  
    if (document.all)
        var xMax = screen.width, yMax = screen.height;
    else
        if (document.layers)
            var xMax = window.outerWidth, yMax = window.outerHeight;
        else
            var xMax = 800, yMax= 600;

    var xOffset = (xMax - imgx)/2, yOffset = (yMax - imgy)/2;
    
    window.open(df,'preview','resizable=no,scrollbars=yes buttons=no, width='+imgx+',height='+imgy+',screenX='+xOffset+',screenY='+yOffset+',top='+yOffset+',left='+xOffset+'');

}