var image_layer = new OpenLayers.Layer.Image( 'Wallpaper', 'images.png', new
OpenLayers.Bounds(-90,-45,90,45), new OpenLayers.Size(318,159), {numZoomLevels:7,
maxResolution:.625} );

var image_layer = new OpenLayers.Layer.Image( 'Wallpaper', 'images.png', new
OpenLayers.Bounds(-90,-45,90,45), new OpenLayers.Size(318,159), {numZoomLevels:7,
maxResolution:.625, **isBaseLayer: false**} );

graphic = new OpenLayers.Layer.Image(
            'Viewshed',
            'http://..../image.png',
            new  OpenLayers.Bounds(20.008697509767,32.076110839844,20.123138427735,32.154235839844),
            new OpenLayers.Size(750, 512),
            { isBaseLayer: false,
              opacity: 0.3,
              displayOutsideMaxExtent: true
            }
        );

radar_overlay = mapObject.addImageOverlay('radar', 'http://radar.weather.gov/ridge
 /Conus/RadarImg/latest_radaronly.gif', 100, -127.620375523875420, 21.652538062803,
 -66.517937876818, 50.406626367301044);