there are two ways: Either you hold the SHIFT key while dropping a file onto the Wonderbrush window. That will put the new image into a new layer. Otherwise, you’ll have to copy the “Bitmap” under “Object” from one canvas and paste it into your other canvas.
Well, it all depends, both behaviours are needed at different times. Best would be to have dropped images open where you have dropped them, as new canvas, new layer, or new object in that same layer. Let’s see if Stippi ever gets to release a WonderBrush NG.