I've never found any good explanation for nightmares.

I know this doesn't sound pleasant but I suggest writing them out in as much detail as you can recall. Treat them like a movie where it's icky but everything is fake. Examine the images for any connection to thoughts you may have in your waking life, and rationalize them. With any luck your mind will eventually realize these images are nothing to be scared of or are not worth displaying because they hold no true value.

When I was a kid I was always bothered by people dying on my favorite TV shows. My mom helped me see that they were actors pretending and they would appear on other shows just fine. It helped me see the disturbing images as an act of pretend and it stays with me today. These days only real life situations can trouble me the way TV violence and disturbing dreams used to.