Versuche es mal so:
index.php:
<div id="menu">
</div>
<div id="slider">
<div id="einblendung"><img src="/images/einblendung.png"></div>
<div id="ausblendung"><img src="/images/ausblendung.png"></div>
user.css:
#slider {
position: relative;
}
#einblendung {
position: absolute;
z-index: 999;
}
#ausblendung {
/* margin-top: 600px; */
position: absolute;
z-index: 999;
bottom: 0;
}