Just a quick follow-up:

Alternatively, you can set the BackgrounImage property of the Style of the respected appearance type of the header (collapsed, hover, expanded), as shown in the WindowsXP Style example in the online browsers site here:

http://samples.infragistics.com/2007.3/webfeaturebrowser/default.htm

e.g. 

    <HeaderAppearance>
        <CollapsedAppearance ExpansionIndicatorImage="./images/downarrow_sel.gif">
            <Style BackgroundImage="./images/XP_Selected.gif"></Style>
        </CollapsedAppearance>
        <HoverAppearance ExpansionIndicatorImage="./images/upArrow_sel.gif">
            <Style BackgroundImage="./images/XP_Selected.gif"> </Style>
        </HoverAppearance>
        <ExpandedAppearance ExpansionIndicatorImage="./images/upArrow_sel.gif">
            <Style ForeColor="white" Height="33px" BackgroundImage="./images/XP_Selected.gif"></Style>
        </ExpandedAppearance>
    </HeaderAppearance>

You can try using the HeaderApearance collection of the Group instance and play with the CollapsedApperance / ExpandedApperance settings - for example the LeftCornerImage/RightCornerImage properties in particular. Please, take into account that the path where these images are taken from is the path to your ig_common files are deployed.

Example: 

    <iglbar:UltraWebListbar
    ID="UltraWebListbar1"
    runat="server"
    ...>

        <Groups>
            <iglbar:Group Text="Group">               
                <HeaderAppearance>
                    <CollapsedAppearance LeftCornerImage="someLeftCornerImage.gif" RightCornerImage="someRightCornerImage.gif" />
                    <ExpandedAppearance LeftCornerImage="someLeftCornerImage.gif" RightCornerImage="someRightCornerImage.gif" />
                </HeaderAppearance>
            </iglbar:Group>
    </Groups>

    ...
   
     </iglbar:UltraWebListBar>