It is important to note that from the 1500 release this class has been re-purposed to manage both map item image overlays and foreground, screen image overlays:

• Five new members have been added. Use either the AddToBottom or AddToTop method to add ImageOverlay objects to the collection. Use the Remove method to delete a specific ImageOverlay or use the Clear method to empty the entire collection.
• The ArcGIS Explorer application now manages two instances of this class and cannot be explicitly instaniated. The collection storing the map item image overlays can be accessed using the ImageOverlayDrawingOrder property whereas the collection storing the temporary, screen overlays uses the ForegroundOverlays property.
• For more information, see the ImageOverlay class.

This class exposes several methods to change the drawing order of image overlays in the map: you can move a specified overlay up or down by a single position in the order, move the overlay to the top of the order (drawing above all other overlays in the map) or move the overlay to the bottom of the order (drawing underneath all other overlays in the map). It is also possible to move an overlay to a specific draw index or obtain the draw index for a specified overlay.

For map items, when an ImageOverlay object is added to the ChildItems collection, the object will be drawn on top of other image overlays in the map, at a draw index of 0.