Yes I know that your attack on this won't work for me since it demands a login. Just gave it a try and ask if someone else was in my situation and had digged deeper than me and stumbled upon a solution. So far it's just a few networks that use these scrambled URLs so it's not that time consuming at the moment to fix the thumbs but if more of them starts with it it will become a hassle
My goal is the highest possible resolution if not 1920x1080. I use Yatse on my tablet where the thumb is used as background for every episode and 400x255 looks terrible and if not for that I wouldn't bother with it.