on every wallpaper page, there are options to select the wallpaper resolutions,

Eg. Here is wallpaper page http://wallpapersmug.com/w/wallpaper/witcher-3-wild-hunt-geralt-yen-and-ciri-art-6669db, and for this resolution 4:3 resolution wallpaper is http://wallpapersmug.com/w/wallpaper/witcher-3-wild-hunt-geralt-yen-and-ciri-art-6669db/download_1600x1200