John Mueller of Google said in a video hangout this morning every 1 hour, 5 minutes, that when you link to images, you need to make sure that those images contain at least some alt text. He said that this "would give us the anchor text" for that link.
The question was "Image links versus regular links. Because we want us to link to the other websites that we have, but show the logo so that it is more user-friendly and that people can see it and not just have a text link to that domain name. Would it still be good to have the link in the image? "
John Mueller replied: "Of course. What I think is useful is that you also have the text link or that you at least use the alt attribute for the image to give us the anchor text that you want to link to that link."
Google has been telling webmasters for as long as I can remember putting alt text on images, but I don't think I ever hear Google say it can act as anchor text, as you would for normal hyperlinks.
Here is the video embed:
Forum discussion on twitter.