From the blog:
Create the appearance of a single image by layering multiple images on top of each other. The pieces can slide around the page as its width is changed, making it a perfect technique for liquid layouts.
This tutorial is slightly adapted from Chapter 9 of my book, Flexible Web Design: Creating Liquid and Elastic Layouts with CSS. You can download most of Chapter 9 for free as a PDF at the Flexible Web Design companion site, as well as download the HTML, CSS and image files that go along with this tutorial.
Since the area available for an image to display within a flexible layout changes on the fly, your images may need to as well. While fixed-width images can work within flexible layouts—as long as they’re not too large, or you have matching minimum widths in place—there are lots of ways you can dynamically change the screen area that an image takes up.
I’ve already written about how to make images literally scale and how to use variable cropping on images. But perhaps you don’t want either end of your image dynamically cropped off—there may be important content on each side that you want to always keep in view. And perhaps you don’t want to scale the entire image, because although this would keep the entire width of the image always visible, it would also change the vertical space the image takes up, which might not work for your design. This is the perfect time to try using what I call a composite image.
Creating what appears to be a single image out of multiple pieces that slide over and away from each other takes a little more work on the graphics side than the variable width image techniques we’ve gone over so far. You can use an unlimited number of images, but we’ll keep it simple and use two for our own alieninvasion example as well.