Fair use of short clips from movie that I bought
IANAL, but I'd guess that if you have to circumvent any DRM just to copy clips of the movie (which you probably do), then it's probably not OK.  Try taking a screenshot of the movie playing in iTunes (ctrl-shift-3 or ctrl-shift-4 on mac).  If it shows up black on your desktop, or if you get an error message, then you probably don't have rights to share it.  But like I said, I'm just guessing.