They really wanted the WPA look for most of these. Though the looking forwards to the horizon look I think is based on some of the war posters of the era (a selected few probably). Although this doesnt explain all the art assets being flipped, the Red Fort for example was flipped for no real reason (the change is minimal).
About the lights being consistent, it may aply, but the lights are all over the place in Civ V assets (I've just checked a few). That lights and perspective can be used for the narrative of an image it's true, and it applies, but doing so with images found on Google gives the least amount of control over the illustration (there's a vital sketching process that was aparently skipped).
A few examples of WPA era posters
Too many
glowing effects is usually considered just an unimaginative solution within the industry (same goes with the rising sun in most/all of them, why so many rising suns?). Too many units are blinging everywhere, likely because the clock was ticking, and they needed something that would give uniformity to all their art assets (so they stuck with the most consistently quick solution they could find... yet it is very consistent at that, which shouldn't be understimated). This in turn killed whatever original inspiration they originally stated they had. WPA posters did not abused glow or the rising sun background, these are all recent fads (and already old fads at that. My own avatar, done 6 years ago, included).
One of them shiny horses of old age...
Although for anyone working on Civ V art assets on a mod, there are 3 things that are vital to the Civ V look (besides google image search): colors (usually primary colors for the foreground and tertiary colors for the background), the glowing from one angle and the rising sun in the background. These are much harder to control than what they sound like though, but if anyone feels their assets are lacking that Civ V kick, try any or all of the above.
Continuing with the subject: though there was an aparent interest at first to follow WPA posters visual codes, the end result just wasnt as consistent in practice. The illustration styles end up being all over the place when compared side by side (from WPA-style mixed with bling and rising suns, Oil-like illustrations done in Painter and comicbook-like ones done by satan).
I may sound pretty mean at it, but it's not just because Firaxis is tottally stuckup and won't go out with me. But because, despite the huge work load, this is a bit of a dreamjob for any illustrator and it's disapointing that the end result was not as original as much of the game was. Illustration is a profession not only of problem solving, but also centered in originality.
. He said that this temple image doesn't count as popular, that's the essence 
, if it's a well known thing, then you don't have much artistic freedom. But that also doesn't give anyone the permission to copy stuff.
