The best resource I've found for analyzing color contrast to WCAG standards is at the Juicystudio website.
There's a Firefox extension for analyzing color contrast in a design, though it's considered beta, and you'll need to register with Mozilla before installing it. It can't do much to test color contrast over images, so you'll need to consider that issue separately in your design. In case you're wondering, we are targeting a pass at the Triple-A conformance level. That's the goal; it may be difficult in practice in all situations.
http://juicystudio.com/article/colour-c ... ension.php