How to replace the icons in speedbar

Question

The icons in speedbar are old-fashioned, I'm trying to replace them, but cannot locate any png files in /home/user/.emacs.d, are they pictures? If they are, where to find them?

And how can I replace them by specifying new image in emacs init file not actually replace them on disk.

Update: It works after adopting the answer in the solution, and it is also easy to switch to a png format picture. To load a new picture other than the original ones, first put a png pic in .emacs.d, and then add the following lines to the init file:

 (require 'speedbar)
 (defezimage ezimage-directory-plus-new
    ((:type png :file "~/.emacs.d/dir-plus-new.png" :ascent center))
    "Image used for empty directories.")

  (setq speedbar-expand-image-button-alist
        '(("<+>" . ezimage-directory-plus-new))

Now my speedbar looks like:

Answer 1

The images are XPM located in a directory such as .../Emacs.app/Contents/Resources/etc/images/ezimage [Location example is for Emacs built --with-ns.] This answer contemplates changing the current XPM images with different XPM images. It is certainly possible to rewrite the speedbar.el library and related libraries to insert PNG images (a time-consuming undertaking), but that is beyond the scope of this XPM answer.

The XPM images can be customized by modifying the values in the variable speedbar-expand-image-button-alist and by adding additional XPM images to the default location. The default values are as follows:

'(("<+>" . ezimage-directory-plus)
  ("<->" . ezimage-directory-minus)
  ("< >" . ezimage-directory)
  ("[+]" . ezimage-page-plus)
  ("[-]" . ezimage-page-minus)
  ("[?]" . ezimage-page)
  ("[ ]" . ezimage-page)
  ("{+}" . ezimage-box-plus)
  ("{-}" . ezimage-box-minus)
  ("<M>" . ezimage-mail)
  ("<d>" . ezimage-document-tag)
  ("<i>" . ezimage-info-tag)
  (" =>" . ezimage-tag)
  (" +>" . ezimage-tag-gt)
  (" ->" . ezimage-tag-v)
  (">"   . ezimage-tag)
  ("@"   . ezimage-tag-type)
  ("  @" . ezimage-tag-type)
  ("*"   . ezimage-checkout)
  ("#"   . ezimage-object)
  ("!"   . ezimage-object-out-of-date)
  ("//"  . ezimage-label)
  ("%"   . ezimage-lock)
  )

---

For example, let us assume that we want to change the directory plus/minus images for the box plus/minus images, and visa versa:

(setq speedbar-expand-image-button-alist
  '(("<+>" . ezimage-box-plus) ;; previously ezimage-directory-plus
    ("<->" . ezimage-box-minus) ;; previously ezimage-directory-minus
    ("< >" . ezimage-directory)
    ("[+]" . ezimage-page-plus)
    ("[-]" . ezimage-page-minus)
    ("[?]" . ezimage-page)
    ("[ ]" . ezimage-page)
    ("{+}" . ezimage-directory-plus) ;; previously ezimage-box-plus
    ("{-}" . ezimage-directory-minus) ;; previously ezimage-box-minus
    ("<M>" . ezimage-mail)
    ("<d>" . ezimage-document-tag)
    ("<i>" . ezimage-info-tag)
    (" =>" . ezimage-tag)
    (" +>" . ezimage-tag-gt)
    (" ->" . ezimage-tag-v)
    (">"   . ezimage-tag)
    ("@"   . ezimage-tag-type)
    ("  @" . ezimage-tag-type)
    ("*"   . ezimage-checkout)
    ("#"   . ezimage-object)
    ("!"   . ezimage-object-out-of-date)
    ("//"  . ezimage-label)
    ("%"   . ezimage-lock)
    ))