The image you choose should reflect the core topic of the website it will be used on. As the topic often cannot be depicted directly,
think of a symbolic representation.
If you have access to online stock images and a program to edit vector images (i.e. Adobe Illustrator), consider choosing a vector image that you can then further modify.
A radial gradient with the lightest part directly behind the illustration can help to emphasize it. Using shades of the main colours of the website creates an overall harmonious impression.