Imported .png images very frequently go "black" with the upper-left quarter of the image still visible, but the other 3/4 of the image blacked out. What's the deal with that? Easily reproduced by importing a .png, saving the file, drawing an object, then choose File > Revert. The image blacks out and the only way to restore it is delete the image, re-import, re-transform. This annoyance happens randomly when opening files as well.