HTML and CSS Reference
Browsers Cache All Images
One of the benefits given by proponents of the sprite method is the load
time of the images (or in the case of mega sprites, the single image). It's
argued that a single GIF image comprising all the necessary image states
will be significantly lower in file size than the equivalent images all sliced up.
This is true. A single GIF image has only one color table associated with it,
whereas each image in the sliced GIF method will have its own color table,
adding up the kilobytes. Likewise, a single JPEG or PNG sprite will likely
save on file size over the same image sliced-up into multiple images. But is
this really such a significant benefit?
By default, image-based browsers will cache all images — whether the
images are sprites or not. So, while it is certainly true that bandwidth will be
saved with the sprite technique, this only occurs on the initial page load, and
the caching will extend to secondary pages that use the same image URLs.