This test determines if your page make use of images which are larger than necessary. Image files often contain metadeta and other information which is never displayed and can be stripped out to reduce the image filesize. Smaller files will transfer faster and result in a quicker page load time.
Checking for image crunchability
This test checks to see if any of the page images can be crunched to save filespace and increase transfer time. If crunchable images are being used then the test reports a FAIL.
If you receive a FAIL for this test, you should consider researching image crunching techniques.
The following online services can be used to optimise images:
This test is based on a YSlow best practice. See http://yslow.org/faq/ for more details.