Images can be a really effective, low prep way of getting students interested in a subject. All you need to do is show them a picture and get them to discuss their reaction to it. With the right choice of photo, you might even get a full lesson out of it.
I like to collect images which I think will be thought-provoking, challenging and perhaps a bit mysterious. Below is a collection of some of these images which you might find useful, along with their back story so you know the context behind the picture.
You can download a powerpoint with some of these images and discussion questions here.