Images and graphics must be provided by the client. Ideally, you have headshots, company photos, or images from your old website to start with. However, for new websites, or even to refresh an old website, KMS recommends a new batch of images and photos from time to time.
Recommended sizes: a minimum of 2,000px for the width. In general, a minimum height of 2,000px is good also.
Backgrounds for images should be true white or even better, "transparent".
Logos: try to get a version of your logo with a transparent background.
For more ideas and sources of where to find and purchase stock images: