As in, the Western film genre. These are depictions of the Wild West and its characters.
(This is not a tag of Western artwork, such as American movies or superhero comics.)
This tag is mostly used on pictures having the iconic cowboy hat and other appropriate leather clothing items (particularly boots, chaps, and vests). If you can see guns in the picture, they are usually revolvers (Colt Single Action Army usually) or rifles (Winchester model 1894 usually). Native American-styled characters might also apply in tagging Western artworks.