laimas,
может так?
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.window:before {
background-image: url(http://placekitten.com/1200/900);
}
.window:after {
background-image: url(http://placekitten.com/1000/750);
}
.window
{
width: 400px;
height: 300px;
position: relative;
border: 1px solid green;
}
.window:after, .window:before{
content: "";
position: absolute;
top: 0px;
right: 0px;
bottom: 0px;
left: 0px;
background-size: cover;
opacity: .85;
transition: opacity 0.4s ease-in;
}
</style>
</head>
<body>
<div class="window" ></div>
<script>
window.setTimeout(function() {
document.styleSheets[0].insertRule('.window::after{opacity: 0}', 4);
}, 1000)
</script>
</body>
</html>