Hi Mike,

The approach I would use is through a style event setter for the ComboCellControl.  You would create a style that targets ComboCellControl and add an EventSetter to handle the MouseEnter event.  Then inside the event, you can check the Content of the ComboCellControl for whether its an Image or not.  If its an Image then you can perform your logic for displaying a larger popup.  The code would look something like the following. 

<ig:XamMultiColumnComboEditor>
    <ig:XamMultiColumnComboEditor.Resources>
        <Style TargetType="{x:Type ig:ComboCellControl}">
            <EventSetter Event="MouseEnter" Handler="OnMouseEnterHandler"/>
        </Style>
    </ig:XamMultiColumnComboEditor.Resources>
</ig:XamMultiColumnComboEditor>

void OnMouseEnterHandler(object sender, MouseEventArgs e)
{
    if ((sender as ComboCellControl).Content is Image)
    {
        // popup logic here.
    }
}