Supposing I\'m setting a background image for a web page in CSS like this:
body {
font-size: 62.5%; /* Resets 1em to 10px */
font-family: Verdana, Arial, Sans
I've already posted the solution in a duplicate question, but for anyone that may require this information I'll post it here as well.
As far as I am aware it is not possible to put it in the same layer, but it is possible to put several images in separate div's on top of one another, and has been implemented by popular usability testing website Silverback (check the background to see how it has been layered). If you look through the source code you can see that the background is made up of several images, placed on top of one another.
Here is the article demonstrating how to do the effect can be found on Vitamin. A similar concept for wrapping these 'onion skin' layers can be found on A List Apart.