I could imagine, that in the USA religion plays a bigger role than in germany. I know a german platform, which gave out their top-tags which were searched (customers, which searched pictures for their wall)...
and there where the top ranked things like: nature (TOP!), flowers, forrest, beaches, cats, dogs. I think photos of the nature are that, what has the best chances. its really neutral, has often no deeper meaning, it exists to look good, and many private customers are searching for things, which just look good and calming at their wall. but - even though, every platform has a little bit other composition of customers and also the offered print-products and the subtile communication of the webdesign (built up more alternative or more mainstream-deco) decide, which target groups buy.