Anyone object to removing Image.replaceWith? I see four problems with keeping it around:
1) Inconstancy: It fails when you try to replace the guts of a CanvasImage with another image in Flash mode, but not in other modes. This could be tricky to make consistent on every platform, as XCanvas is not always a subclass of XImage, and we can't guarantee that passing the guts of a CanvasImage back and forth with non-canvas Images is safe.
2) It's buggy: it doesn't notify the associated image layers that its guts have been replaced, which means that it doesn't currently work in DOM mode. I suspect that it's not currently re-creating textures in WebGL either, but I haven't tested it.
3) It's nearly a duplicate of the functionality of ImageLayer.setImage()
4) It makes images not-quite-immutable, which makes it somewhat more difficult to reason about them.