Its earliest usages make the most sense to me, being used on things resembling "ancient civilization." (post #2425208) I'm not sure why random users keep coming along and nuking it from appropriate images, just to add it to completely nonsensical ones, such as user #507781 emptying it before adding it to three random FEater posts.
@tremolo_measure Since you seem to have adopted the tag, can you explain what you're using it for? Some of them have a "sepia" look, but others are just red or yellow toned, and I wouldn't use "ancient" to describe any of them.