Aside from the standard Gravatar, WordPress allows users to choose from a list other default avatars. If you are creating your own theme, you can add an avatar or icon consistent with your theme’s styling to the list of default choices.
WordPress makes this a rather simple endeavor with the
avatar_defaults filter. Add the code snippet below to a callback function (replace themeslug_ with your theme’s function slug) add an image of your choice to the list of avatars users can select from.
Although you can use any size, avatars of 60×60 pixels are the generally accepted size.