If you are looking for a comprehensive online resource for Civil War Union Generals, you need look no further than Wikipedia. They have an amazing resource for Union Generals – including photos/portraits and biographies.
The list of generals is alphabetized and easy to navigate to your favorite general. And with one more click you can get a large size portrait/picture of the Union General that is often in the public domain so you can use it in your own works. Another click will give you the biography of the General.
You can access this resource at Wikipedia: American Civil War Union Generals