If you have trouble coming up with tricks, regulating the light can do a lot to make your picture spooky. You can also blur things out as an alternative. You wouldn't want to add a lot of design since that itself might remove the ominous aura.
That would depend on the images. Are they photos or illustrations? If they are illustrations, you can always use either Photoshop or Illustrator to add some spooky elements like black cats, stars, witches etc.