If images are not labeled with alternative text, individuals without access to the images will not be able to use the site (for example, individuals without sight).
Every image should have an alt tag. Significant images (images with content and/or navigational value) should have alt tags that reflect their content or usage. Insignificant images (eg. spacer images, small decorative & repetitive pictures) should have an alt tag of "".
For more accessibility guidlines, visit the W3C guidelines.
Books on how to create accessible sites:
An image is considered significant by WebEval if the image is bigger than 10x10 pixels or contains a link.
Other more comprehensive evaluations are also available online inculding Cynthia Says , Wave and FAE.