You don't need medicines. Just relax and give yourself time ... it will go .

May I know on what occasion you visualised that incident. Did u read any story or watched some movie or heard some body describing like that.  If u want to come out of that thought u need to replace it with a good incident or thought which u recently liked or enjoyed OR which you want instead.

whatever you imagine or fear of actually gets attracted to you so you should stop thinking or dreaming negative totally.Being busy or occupied with hobbies etc helps or reading positive thinking books

Hi Ruminations of disturbing memories & negative thoughts are quite common for few days after witnessing a traumatic event. You need to be mindful about these experiences & emotions in the context. Practice yoga, relaxation, breathing techniques & distractions by normalising these experiences in the context. However if it is worsening in frequency, intensity or duration & is causing lot of distress, please consult a psychiatrist for appropriate techniques to handle & may even benefit from medicine for sometime. Regards