Here’s an interesting website which contains a series of different images which can be used as writing prompts or story starters. The site is produced by Luke Neff who uses these images with his own class.
Each one involves a thought-provoking image and a question/scenario. Take a look here: http://writingprompts.tumblr.com/
If you want to make your own, then you could use sites such as The Big Picture to find powerful images and combine them with a question of your own.Use the screen capture tool to grab the image and bring it into your IWB software, or copy/paste into PowerPoint.
If you know of other great story starter sites, let me know in the comments.