14 DAY TRIAL // The second video blog post of the Webmaster Central Blog is basically about what a user can do for his peers to have a better experience while using Google's Image Search. It's very important, Google's web spam team's head, Matt Cutts, says, that you accurately describe a picture in the best possible way you can by using alternative tags, or "alt tags" as you might find them talked about.
He chose as an example a regular picture of a cat and a ball of yarn that can either be described by the classical picture tag that every picture has, a.k.a. the picture name or by its alt tag. In the first case, if somebody wanted to search for it specifically it would have had to enter a query looking something like "DSC 00000125.jpg" or something similar to that. Let's be honest and admit that this is no way to find whatever it is you're looking for, unless you are specifically handed the information. But if that happens, why not also hand over the picture as well?
The second case, advertised by the Google team, is that every picture should be tagged with a certain number of words that would most accurately describe it and thus narrow the search results returned by Google. For the cat example, all you'd have to do is add "cat, , yarn, " and things like that, so that the end result would pinpoint that specific picture whenever the tags searched for coincide with it.
As I said before, all this is being politely asked of you by Google, in order to improve your user experience with the various services that the Mountain View based company provides. In the hope that "what you yourself don't like don't inflict onto others" or however that old saying went, it's ultimately up to you to have a good time or a less good time while browsing the Internet.