This test determines if your page make use of image sprites. Grouping multiple images into one image sprite drastically speeds up delivery and load time. When many images are used independently many network requests are required to download them all, an image sprite combines the images into a single file which can then be cut-up using CSS.
Checking for image sprites
This test checks to see if more image files are being directly referenced than necessary. If enough singular image files are being used then the test reports a FAIL.
If you receive a FAIL for this test, you should consider researching image sprite creation techniques.
This test is based on a YSlow best practice. See http://yslow.org/faq/ for more details.