Digital images are protected by copyright, even if you do not see a copyright symbol displayed.
Make sure you check the usage policies on websites before downloading any images.
For all images where permission for use is granted, be sure to cite the source.
Cornell University has a guide to citing images.
Google Images can be used to search for images on the Internet. For example, type "ultrasound" to retrieve ultrasound images. Using one or two search terms usually works best with this image searching site.