I'm a little late to that discussion, and I held off for some days because I wasn't sure if this fits... but I think I have a good idea where Mr. Snyder got his inspiration from.
If you google for 'amazons', you will sooner or later find links to the 'Dahomey Amazons'. The kingdom of Dahomey in western Africa employed a female army during the 19th century. The Dahomeans also had a male army which had superior numbers, however the women were considered to be more proficient, highly motivated, and loyal to the death. The Europeans who had a chance to see them called them 'Amazons', although this is not quite right - 'amazon' implies an all-female or alt least matriarchal society, and the Dahomean society was quite patriarchal. The female soldiers fought for a king who was invariably a man. In their own language, they were called 'ahosi' (literally, 'wives of the Leopard', i.e. wives of the king).
Anyway, if you google for 'Dahomey Amazons', or just look up the wikipedia entry, you will find this picture (which is NSFW, so I'm just giving a link):
https://upload.wikimedia.org/wikiped...ey_amazon1.jpg
It shows a high-ranking officer of the ahosi, whose name is given as Seh-dong Hong-Beh. Note that Diana strikes the same pose, except that Diana holds a spear instead of a musket. (Actually, I can't think of why Diana would have a spear - which never was part of her arsenal - unless it is to recreate this pose.)
In case you wonder about the costume of the ahosi: she is barefoot, because nobody in Dahomey was allowed to wear shoes, except for the king (and foreign visitors). The striped dress was peacetime uniform. In war, they wore brown or grey clothes, and scouts or snipers would wear camouflage suits which they made of grass and foliage (Ghillie suits in modern terms). In peacetime, the ahosi had various duties as police force and executioners, so this is probably the reason why she holds a severed head while in peacetime uniform.
If you want to read more about the Dahomey Amazons, I recommend the wikipedia entry, and some of the books mentioned there: 'Amazons of Black Sparta' by Stanley Alpern and 'Warrior Women' by Robert Edgerton. I would not recommend 'Wives of the Leopard' by Edna Bay (although amazon.com recommends it ) - it mostly covers the economical and political role of women in Dahomey, not the military role. Also, be warned: those books have some bloody and violent descriptions... less 'Wonder Woman' and more 'Red Sonja' in comicbook terms