I always thought that was done using keywords on the page the image was taken from (and image captions, alt text, titles and filenames). This is reinforced by w

I always thought that was done using keywords on the page the image was taken from (and image captions, alt text, titles and filenames). This is reinforced by what you see when you go many pages ahead in images and see things that don't seem related to what you searched, but the keyword is there somewhere on the page.