The series cards are quite small to use full-sized TheTVDB posters, thumbnail URLs (just append a _t before the file extension) should have enough resolution and make the loading which is sort of sluggish (in comparison to v1's banners) slightly faster.
Example:
http://thetvdb.com/banners/series/260586/posters/5e911d95b60d1.jpg
http://thetvdb.com/banners/series/260586/posters/5e911d95b60d1_t.jpg