If you select any Bitmap Object and go to the menu Object, you have enabled the item "Clone Bitmap Object".
This will make a virtual copy of the bitmap object. The new Clone object 'copy' all the visual properties of the parent Bitmap Object.
Now if you change the parent object (Bitmap) for example enabling the Alpha transparency, all the child objects will change as well.
Note: Clone Objects can exist across pages!
On 'Page 1' we copy the Clone Object to the clipboard (Ctrl+C) and here on Page 2 we paste it (Ctrl+V) - we have child of the parent bitmap from Page 1.
If you open the Clone properties, you will see its Parent is 'Page 1::Bitmap'
Knowing the fact the clone objects takes very little space we can make some tricks (animations) with the script and clone objects without creating huge files
Exploring the Clone Object Properties you notice the Clone Objects have some little independency from the parent. They can be independently Shown/Hiden and they each have its own action properties.
See CloneObject.mbd sample