(add-hook 'after-init-hook
          (lambda ()
            (define-key global-map [tool-bar pepper-button]
              '(menu-item "Pepper" mark-whole-buffer
                          :image (image :type xpm :file "/usr/share/emacs/25.3/etc/images/pepper.xpm")
                          ))))

    (tool-bar-add-item "pepper" 'mark-whole-buffer
               'mark-whole-buffer
               :help   "Run fonction mark-whole-buffer")