I am struggling with this right now. Alt text, captions, audio (read to me) options are good. I work with an accounting software app that redesigned its palette to red, black, and gray. No green to signify income. Made no sense to me and their gray text (also very skinny sans serif) made reading many nav bars difficult. Some URLs are accessible from the build-out, but the DIY sites often don't have the same capacity. As with secure sites, I think google will be downgrading those sites with little or no compliance or accessibility add-ons.
The words in the tag should be more than a description. They should provide a text equivalent of the image. In other words, the tag should include the same meaningful information that other users obtain by looking at the image. In the example of the mayor’s picture, adding an “alt” tag with the words “Photograph of Mayor Jane Smith” provides a meaningful description. 
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