Free environmental images
Searching on the web you may find sites which offer free images, but always be careful to check the small print to confirm that you are allowed to use them for your particular purpose.
Always include copyright information and a reference to the site where you downloaded the image.
The best place to start is often http://www.flickr.com/creativecommons/ - images there are offered under various licenses which let you reuse them for free. Be sure to check what type of license your chosen image is offered under, and stick to the conditions of that license.
Similarly, http://search.creativecommons.org/ allows you to search for images with a reuse license. Once again be sure to check what type of license your chosen image is offered under, and stick to the conditions of that license.
If you find an image that you would like to use elsewhere, contact the company/organisation to ask for permission.