There is no way to change that through the public object model; this view was based on the pre-Vista WIndows Explorer which also did not provide any way to change that. If you like you can submit a feature request for the ability to do this.

Note that you can work around this limitation in the meantime using the IUIElementCreationFilter interface (assign an instance of this class to the control's CreationFilter property):

#region ThumbnailCreationFilter
public class ThumbnailCreationFilter : IUIElementCreationFilter
{
    UltraListView listView = null;
    public ThumbnailCreationFilter( UltraListView listView ) { this.listView = listView; }

    #region IUIElementCreationFilter Members

    void IUIElementCreationFilter.AfterCreateChildElements(UIElement parent)
    {
        if ( parent is UltraListViewIconicImageAreaUIElement )
        {
            ImageUIElement imageElement = parent.GetDescendant( typeof(ImageUIElement) ) as ImageUIElement;
            if ( imageElement != null )
            {
                Rectangle rect = parent.Rect;
                rect.Inflate( -4, -4 );
                imageElement.Rect = rect;                   
                imageElement.Scaled = true;
            }
        }
    }

    bool IUIElementCreationFilter.BeforeCreateChildElements(UIElement parent)
    {
        return false;
    }

    #endregion
}
#endregion ThumbnailCreationFilter