Alright, so it turns out I forgot to use a semi-colon in my recursive search and wasn't actually searching the php files... hence teh reason I couldn't find the references to these images.
Changing the icons is actually pretty simple.  Everything you need to reference from a code perspective is in the file:
- include\inc_lib\admin.functions.inc.php
If you want to keep the icons lined up you'll probably want to switch the "X" icon for editting to the "edit_22x11.gif" version and then create a new "add_22x11.gif" icon that is 22px wide instead of 11.  Then change the references in the above file accordingly, and finally make sure to change the width o fthe right column so it can fit the wider icons on one line.  Specifically you'll need to edit the following:
start-----------------------------------------------------------------------------------------------------------
			//       $a .= $at."</a></td>\n</tr>\n</table></td>\n<td width=\"77\">";
			$a .= $at."</a></td>\n</tr>\n</table></td>\n<td width=\"88\" nowrap>";
			//-- 19-11-2004 Fernando Batista end-------------------------------------------------------------------------------------------------------------
Just change 88 to something that accomodates the 11px increase, like 99px or 100px.  All done.  Here's a quick shot of what mine looked like.
 
Best,
Rick