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You'd use alt text on an image link, but not text. Alt text is the text that would display instead of the image if the image isn't shown for whatever reason (e.g. for the visually impaired or for those who prefer to browse text-only). Title text is the text that displays when you hover over something. You can use it in addition to the alt text, but it would be the title that displays when you hover over the image. I'm sure all three (alt, title and anchor text) help Google to learn what a link is all about.