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>