I've only had that rectangle happen when I have moved the bitmap file from its folder, so when Rhino goes to load up the bitmap to display it, the data is no longer where it expects to find it. I think there is a message printed in the rectangle along the lines of 'Bitmap xyz.jpg is no longer available' Are you sure you haven't put the bitmap file in another folder? How big are the bitmap files? I wondered if there might be a memory issue here? You can always use Paintshop Pro or some other bitmap editor to resize big bitmaps, which will help with the memory issues. I tend to reduce the colours to greyscale, or even black/white images, or grey/white png files, for things like hull sections, makes them easier to trace over. Otherwise, ask on the Rhino usergroup forums. I've not had this problem, so don't know what else it might be.
Tim P