Images do a great job of helping written ideas to be understood. Often, writers search for an image that literally depicts they subject they are discussing. However, an image that directly shows your subject may not be as engaging as one that simply reinforces the idea of what you are writing about.
To learn more, check out this Tech-Talk article about thinking outside the box when choosing images to pair with written content. If you are prompted for a username and password, both are eglibrary.