var nivel = 30;
var velocidad = 30;

var i = 99;
var nImg = '1';
var efect = 'off';

function doDark() {
    if ((efect == 'on') && (i >= nivel)) {
	    document.getElementById('img'+nImg).style.filter = 'alpha(Opacity='+i+')'
	    document.getElementById('img'+nImg).style.MozOpacity = '0.'+i
	    i = i-10
	    setTimeout("doDark()",velocidad)
    }

    else if ((efect == 'off') && (i <= 100)) {
	    document.getElementById('img'+nImg).style.filter = 'alpha(Opacity=100)'
	    document.getElementById('img'+nImg).style.MozOpacity = '1'
	    i = 99
    }
}