function openWindow() {
window.open("","Zvetseny_obrazek",'toolbar=0,location=0,scrollbars=1,width=830,height=730,resizable=1');
}

function noveOkno(sirka,vyska) {
var leftpos=(screen.width - sirka)/2;
var toppos=(screen.height - vyska)/2;

window.open("","Zvetseny_obrazek",'toolbar=0,location=0,scrollbars=1,width='+sirka+',height='+vyska+',resizable=0,top='+toppos+',left='+leftpos);
}