There are no properties to change these images, but you can draw your own image with a draw filter. If you have never used draw filters before, the following help article has a brief overview: http://help.infragistics.com/Help/NetAdvantage/NET/2008.1/CLR2.0/html/Win_Draw_Filter.html.

In your GetPhasesToFilter implementation, if the drawParams.Element is a CaptionButtonImageUIElement, return BeforeDrawImage. In your DrawElement implementation, draw your own image and return true to prevent the normal image from drawing. You can determine which button is being drawn in your DrawElement method by getting the parent element, casting it to a PaneCaptionButtonUIElement and checking that element's PaneButton property.