One of my client's website uses one image on all its pages at a very prominent place (just below top navigation bar). My question is can I use different Alt tags for that single image on different pages according to the primary keyword of that page?
Same image with different Alt tags is like - Same content with different titles My advice is don't do it (although there is no harm)
Alt is not a "tag"... It's an attribute... Tags are used to open and close HTML/XHTML elements. That being said, the alt attribute is an accessibility attribute used by screen readers for the blind and also used to for any user agent that cannot display images (text browser like Lynx for instance). The value of the alt attribute should describe the content of the image and not be stuffed with keywords unless your desciption of the image just happens to contain some of the keywords. Remember this is what is going to be read to the blind by the screen reader or show in text browsers or browser used for dialup where the user has turned off images. And that being said, there is nothing wrong with using multiple alt attribute values for describing the same image as long as EACH alt attribute value used accurately describes the image. If you ask 100 people to describe the image you'd likely get 100 answers. If you as the same person to describe the image on different days, they'd likely give you slightly different descriptions. This happens naturally all over the web anyway is the same image gets used by different developers over time. So the engines aren't going to "ding" you for doing so.