Reapply Gerd's change from 2000-09-18, which seems to have gotten lost:

(tool-bar-add-item-from-menu): Like in toolbar-add-item, if image
  doesn't have a mask add a `:mask heuristic'.

lisp/ChangeLog

@@ -11,6 +11,11 @@
 	hardwiring :family.
 	* hi-lock.el (hi-black-hb): Likewise.
 
+	Reapply Gerd's change from 2000-09-18, which seems to have gotten lost:
+	* toolbar/tool-bar.el (tool-bar-add-item-from-menu): Like in
+	toolbar-add-item, if image doesn't have a mask add a `:mask
+	heuristic'.
+
 2000-09-19  Stefan Monnier  <monnier@cs.yale.edu>
 
 	* diff-mode.el: Docstring fixes.

lisp/toolbar/tool-bar.el

@@ -114,6 +114,8 @@ PROPS is a list of additional properties to add to the binding."
 			key kk)))))
       (when (and (symbolp submap) (boundp submap))
 	(setq submap (eval submap)))
+      (unless (image-mask-p image)
+	(setq image (append image '(:mask heuristic))))
       (define-key-after tb-map (vector key)
 	(append (cdr (assq key (cdr submap))) (list :image image) props)))))