The American Battlefield Trust is a nonprofit organization devoted to the preservation of Civil War battlegrounds. They have also created virtual tours of 20 Civil War battlefields.
The NPS website offers a wealth of information about the places, people, and events of the American Civil War.
Unknown author; Kurz & Allison, Art Publishers, Chicago, U.S.A. / Restored by Michel Vuijlsteke [Public domain], via Wikimedia Commons
The Photographic History of The Civil War in Ten Volumes: Volume Two, Two Years of Grim War. The Review of Reviews Co., New York. 1911. p. 90.
CC BY-SA 3.0 by Conaughy at English Wikipedia
Ken Lund from Las Vegas, Nevada, USA [CC BY-SA 2.0], via Wikimedia Commons
"Battle of Chattanooga" by Thure de Thulstrup (1880).
Waud, Alfred. "Scene of the explosion Saturday July 30th" Library of Congress Prints and Photographs Division. http://hdl.loc.gov/loc.pnp/cph.3g10794
Image from page 342 of "The photographic history of the Civil War : in ten volumes" (1911), via Flickr.
Field works at Franklin, Tenn., occupied by the 23d and 4th corps during engagement of Nov. 30th , Maj. Gen. J. M. Schofield, comdg. [S.l, 1864] Map. .
"Havoc," effects of a 32-pound shell from gun of Second Massachusetts Heavy Artillery. Confederate caisson and eight horses destroyed. Fredericksburg, May 3, 1863. Capt. Andrew J. Russell. Mathew Brady Collection. [Public domain], via Wikimedia Commons
"View from the summit of Little Round Top at 7:30 P.M. July 3rd, 1863", painting by Edwin Forbes
"Sketch of the Third Brigade, First Division, Sixteenth Corps at the Battle of Nashville, Tennessee" by George H. Ellsbury, December 15, 1864.
Battle of Perryville image from Harper's Weekly (November 1, 1862)
Hospital scene behind the lines of Pickett's Charge, Gettysburg, Pa. Boston Public Library CC BY via Flickr
Sketch by Alfred Waud of a ship at Deep Bottom
Sketch of Second Battle of Fort Fisher, appeared in Harper's Weekly, February 4, 1865. page 72
Kurz & Alison, "Battle of Stone River." Library of Congress [Public domain]
Painting of H.L. Hunley submarine by Conrad Wise Chapman (1864)
The Dred Scott decision : opinion of Chief Justice Taney, [Public Domain] Library of Congress, via picryl.
![Map by Hal Jespersen, www.CWmaps.com [CC BY 3.0] via Wikimedia Commons.](https://commons.wikimedia.org/wiki/File:First_Bull_Run_July18.png){kind=link}
![Barlow's Knoll at Gettysburg [Public domain], via Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Barlows_Knoll_after_first_day%27s_battle,_Gettysburg,_July_1,_1863.jpg){kind=link}
!["The Monitor and Merrimac: The First Fight Between Ironclads" by Jo Davidson [Public Domain] via Wikipedia.](https://en.wikipedia.org/wiki/File:The_Monitor_and_Merrimac.jpg#metadata){kind=link}
![Sneden, Robert Knox [Public domain], via Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Plan_of_the_Battle_of_Kernstown,_Va._March_23rd_1862._LOC_gvhs01.vhs00077.jpg){kind=link}
!["Historical Marker", by Brian M. Powell [CC BY-SA 3.0] via Wikipedia.](https://commons.wikimedia.org/wiki/File:Battle_of_Rich_Mountain-Historical_Marker.jpg){kind=link}
![Grant from West Point to Appomattox, by Thure de Thulsrup. [Public Domain] via Wikipedia.](https://en.wikipedia.org/wiki/File:Ulysses_S._Grant_from_West_Point_to_Appomattox.jpg#file){kind=link}
![Jed. Hotchkiss [Public domain], via Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Battle_of_the_Wilderness_1864_map.png){kind=link}
![William Marsh, Springfield, ILJulian Vannerson [Public domain], via Wikimedia Commons](https://commons.wikimedia.org/wiki/File:Lincoln_Douglas.jpg){kind=link}