[racket] Windows image bug (again, sorry)

So, Windows seems to be able to open files with modified bitmaps, but
it's not happy about saving or pasting them.

Try this:

Insert an image.
Crop it in the Interactions window.
Try to copy and paste that image back up to the top.
This is on the latest nightly build.

What's worse, it tries to save the file, gets an error, and destroys
the file that was already there. My poor 7th grader just lost 2 hours
of work. :-(

I know Robby's busy, so we'll just be careful until he has time to put up a fix.

Todd