A Google Groups thread has discussion around how to properly use the alt attribute and title attribute for your web page images. Of course, Google recommends that you make sure to make them as useful for the end users. So use them as per W3C recommendations but let's hear from Google on what they expect.
JohnMu from Google explained how to use both the alt and title attribute:
- alt attribute should be used to describe the image. So if you have an image of a big blue pineapple chair you should use the alt tag that best describes it, which is alt="big blue pineapple chair."
- title attribute should be used when the image is a hyperlink to a specific page. The title attribute should contain information about what will happen when you click on the image. For example, if the image will get larger, it should read something like, title="View a larger version of the big blue pineapple chair image."
I'll quote John from Google now:
As the Googlebot does not see the images directly, we generally concentrate on the information provided in the "alt" attribute. Feel free to supplement the "alt" attribute with "title" and other attributes if they provide value to your users!
So for example, if you have an image of a puppy (these seem popular at the moment :-)) playing with a ball, you could use something like "My puppy Betsy playing with a bowling ball" as the alt-attribute for the image. If you also have a link around the image, pointing a large version of the same photo, you could use "View this image in high-resolution" as the title attribute for the link.
There is a Google blog post on using alt attributes as well.
Forum discussion at Google Groups.