Howto exchange the expand/collapse icon of a groupnode with a full row height version

Hi Isomorphic,

I have a ListGrid and want to change the design of a groupNode's expand and collapse buttons.
Files are called "ListGrid/group_opened.gif" and "ListGrid/group_closed.gif" (both 16px * 16px).

I did not find a reference to the file names in the skin's css or js. Could you give pointers on how to exchange these icons against full row-height versions (30px * 30px, no transparency) with no borders/margin/padding? -> The row's background is only visible after right of the icon.

Thank you & Best regards,
Blama