Those icons are dynamically set via client-side javascript each time you expand/collapse.  If you don't want to change the icons themselves, you'll need to redefine the relative links to the icons by overriding it with your own links.

As a test, try adding this to your page (adding it to the SOURCE of a Text/HTML module will work fine):

< script >
   document.getElementById('IconBar.ascx_imgVisibility').src="http://www.eguanasolutions.com/portals/0/blog/images/generic/pm-plus.jpg";
   document.getElementById('IconBar.ascx_imgVisibility').max_icon="http://www.eguanasolutions.com/portals/0/blog/images/generic/pm-plus.jpg";
   document.getElementById('IconBar.ascx_imgVisibility').min_icon="http://www.eguanasolutions.com/portals/0/blog/images/generic/pm-minus.jpg";
< /script >

NOTE:  Remove the extra space just after the "<" and just before the ">" in the opening and closing script tags.

If that does the trick, just add it to the bottom of your skin.  You might also consider forcing the the code to run after the page has loaded to be sure it makes the changes AFTER the code is all in place.  One easy way to do this in DNN from javascript (using "closure") is detailed here:
 
  http://www.eguanasolutions.com/DNN_Blog/EntryID/4.aspx

Cheers!
-mamlin