Alt attributes are for users, not search engines. If you're purposely writing alt attributes for search engines then you are providing non-natural alt attributes which, in turn, lowers user experience and sends bad signals to Google/Bing, search engines want content optimised for users, not search engines. Look after your users and the search engines will look after you.
Good usage of the ALT attribute is composing the image ALT value that
consists of a short description that informs users the context of the
image. Sometimes describing less is often more and other times more is
more! Your descriptions should contain the elements of the image that
you want them to visualise, it is not for telling Google to rank THIS
because of this KEYWORD. Treat the ALT tag as a solution for partially
impaired, completely impaired or for users that have disabled images
in their browsers.