CSS Sprites
One of the most exciting techniques to emerge from the CSS community in recent months involves using a single large background images as a container for several smaller images, then using the CSS background position properties to show different parts of the larger image in different places (and different hover states) around a page.
To my knowledge, the first example of this trick in use was Petr Stanicek’s Fast rollovers, no preload needed demo. It has since been expanded upon by other members of the community: Dave Shea’s A List Apart article CSS Sprites: Image Slicing
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