Photograph of William McKinley, 1865
Matthew Brady
"Major McKinley: William McKinley and the Civil War", by William Armstrong (2000), but previously published at least as early as 1901 in "The life of William McKinley" by Oscar King Davis and John Kimberly Mumford -- see <a rel="nofollow" class="external text" href="https://books.google.com/books?id=EY9DAAAAYAAJ&amp;">here</a>, p. 4.