One hundred years ago today, on January 31, 1917, Governor Robert D. Carey signed the state flag bill into law and the bison flag was officially adopted for the state of Wyoming. Twenty-three year old Verna Keays of Buffalo, Wyoming made the winning design seen here. (Photo from Wyoming State Museum on Facebook.)
The Senate vote was 22-0 with 5 absent and the House vote was 53-1 with 3 absent. There was a lone dissenter from Goshen County. The Session Law (Chapter 8 of 1917) can be found using Hein Online at gowyld.net/govtinfo.html.
A search of Wyoming Newspapers, using the search: bison and flag, yields stories in the Rock Springs Miner, Wyoming Tribune, Guernsey Gazette, Buffalo Voice, and more. These stories appear in the early February editions of 1917.
From the Rock Springs Miner, February 17, 1917, page 1:
The Great Seal of the state is the heart of the flag, this is within a white American bison, the monarch of the plains. The blue wbich comprises the larger part of the flag, Is pre-eminently the color of our skies and symbolic of fidelity and justice. There is a narrow line of white surrounding the field of blue, this stands for freedom of the plains and purity over all. All this is bounded by a margin of red, and signifies the red men of Wyoming and the blood of the pioneer.[/su_quote]
There is a PDF of the flag that can be printed out and colored in CultureGrams.
All of these sources can be found in GoWYLD.net.