I've got this image and i want to put it over my website as a foregound. It should overlap the background and the elements. I'm using CSS.
If I understand correctly you want something like this:
Html (place somewhere on top):
<img src="URL_TO_IMAGE" id="overlay"/>
CSS:
#overlay{
width:100%;
position:absolute; /*You may also use fixed depending on your needs.*/
left:0;
top:0; /*If you want it at the bottom you probably want bottom:0 instead*/
z-index: 999;
}