I just took a look at the code that's returned when you open https://www.youtube.com/embed/<videoid>?feature=oembed and noticed a couple of additional image file names you may want to check.

        iurl: "default.jpg",
        iurlmq: "mqdefault.jpg",
        iurlhq: "hqdefault.jpg",
        iurlsd: "sddefault.jpg",
        iurlpop1: "pop1.jpg",
        iurlpop2: "pop2.jpg",
        iurlhq720: "hq720.jpg",
        iurlmaxres: "maxresdefault.jpg"

        120: "default.jpg",
        320: "mqdefault.jpg",
        480: "hqdefault.jpg",
        560: "pop1.jpg",
        640: "sddefault.jpg",
        854: "pop2.jpg",
        1280: "hq720.jpg"