It depends on what you want to do with the images (i.e. whether you want to use them for commercial or non-commercial purpose). Many online databases will allow you to use images for private study or non-commercial purposes, but will ask you to contact them if you want to use them commercially.
I use some databases for teaching medical students, I can share some of the databases/websites I use.
Radiopaedia is one of the larger databases, it contains X-ray, CT, MRI and ultrasound images, many have descriptions and are incorporated into cases. There are conditions in how you can use the images, you will need to read the licensing page for details.
http://radiopaedia.org/encyclopaedia/all/all
The site below is focused on emergency ultrasound, but it does contain images and descriptions of viscera. I used parts of this site in the past for teaching anatomy to medical students, they generally found it quite helpful:
http://www.sonoguide.com/introduction.html
This site is case-based, it has multiple ultrasound images, including images of viscera. It also has some powerpoint slides and scanning protocols:
http://www.ultrasoundcases.info/Default.aspx
This site has multiple ultrasound images with some links and descriptions. I don't find it as extensive as some other sites personally (it also has advertisements), but it does have some interesting images:
http://www.ultrasound-images.com/index.php
This site requires paid membership to see all resources, but some are accessible to general public (eg. normal ultrasound images):