This can be soooo hard and seem counterintuitive if you haven't heard it before - but try allowing them to be there. Rather than trying to get them to stop or trying to activly get them to go away, try to let them be there. Try to witness it without judgment and you can try first by defusing from the thought a bit. Knowing that you are HAVING a thought - you are HAVING an image. Realizing eventually that this doesn't necessarily mean anything about you or what's going to happen but just that our brains go haywire sometimes.