bug#7480: 24.0.50 no images in Custom

[Jonas Bernoulli <jonas@bernoul.li>]

In Custom buffers no images are shown for widgets that support it;
instead the respective text representations are shown.
`widget-image-enable' is t and the required images can be found.

I verified that `widget-image-insert' actually does get called when
appropriate and likewise calls `image-insert' .

Evaluating (insert-image (widget-image-find "right") "text") in the
scratch buffer does insert the image there. Setting
`before-change-functions' nil an evaluating the same expression in
Custom buffers also works.

In Custom buffers (text-properties-at (point)) returns

(rear-nonsticky
 (display)
 display
 (image :type xpm :file
"/usr/share/emacs/24.0.50/etc/images/custom/right.xpm" :ascent
center))

on "Show Value" as will as the manually inserted "right" image. I
don't understand in what way they differ that makes them be shown as
image resp. text.

Thanks

-- Jonas