Why is Google Image Search Important To Me?
As mentioned, Google Image Search gets more traffic than supposed major search engines in the UK, typically getting more visits than websites such as Amazon and MySpace (Source: Hitwise). This means Google Image Search is a potential source for traffic to your site, and although perhaps not as targeted as conventional search, implies that people are actively searching for what you provide.
The biggest reason to start optimizing for Google Image Search is that it is not as competitive as normal search, despite the great amount of traffic it can provide. Although ranking well in Google Image Search itself does not help your conventional search engine rankings, a by product of ranking well in Google Image Search could mean this will happen eventually because of the viral potential.