at most four images can be used as overlay masks怎么理解？
Identifies an image in the list as an image that can be used as an overlay mask.
Use Overlay to assign an overlay index to one of the images in the list. Once an image has an overlay index, it can be used as an overlay mask. An overlay mask is an image drawn transparently over another image.
ImageIndex identifies the image that can act as an overlay mask.
Overlay is the overlay index to assign to that image. It must be a value between 0 and 3 (inclusive). Note that this means at most four images can be used as overlay masks.
If successful, Overlay returns true.
To draw an overlay mask over an image use the DrawOverlay method with the index of the overlay mask.