The photo can be found on the next url with as description source US Naval Center Photograph (PD-USGOV). Its’ a reproduction of the engraving published by Endicott&Company at New York, 1865.
http://en.wikipedia.org/wiki/File:USS_Dunderberg_(1865).jpg
Thanks to the fact that nowadays more and more books are digitized we are able to read books that are some times for decades no longer available for the public for several reasons. That's quite a pity while these books contains useful information while the archives are destroyed, incomplete or nor accessible. Main compared the British navy with the major naval powers at that moment in the world like France and the United States.
P. 62: “Next to France in importance is the American navy. In December, 1864, it consisted of 65 vessels, of which 71 were armour-plated. This navy had been constructed with remarkable rapidity, for since the year 1861, 141 steamers and 62 iron-clads had been built. The most remarkable addition during last year was the Donderberg which was launched at New York on 22nd June, 1865. Her floor is flat and her sides are angular; her hull is surrounded by a casement pierced for twenty-one guns, which is 3 feet thick, and covered with iron 5½ inches thick. She is then, as it were, two vessels, the space between her hull and this casement being used for coal bunkers. She is nearly 400 feet long, about 70 broad, and her tonnage is 5,090. Her armament will be four 15-inch Rodman guns, and from twelve to fifteen 11-inch Dahlgren gun, and her speed is expected to be 15 knots an hour.
Source
Robert Main. “The Navy in 1865” in: British army & navy review. Vol IV, London 1866.
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