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If you referring to the ALT tag, for accessibility sake and in the event the graphic doesn't download, you should name the graphic what it is: Home, About Us, Site Map, etc. I cannot attest to how SE's would consider the ALT if it's a keyword, but this is one instance where you should think of the visitor's needs. Just my opinion :)
And then view the source of the page. I think that it shows that the normal google search does not use the name of the graphic, but it does count alt text.
However the Google image search [images.google.com] does use the graphic name.
Not sure about the other engines. I can't see it doing you any harm though.