Some series are full of scenes that can be turned in wallpapers right away, since some of them are pretty old and lack huge sizes I think the DVD would contain the highest resolution available like how Sayonara Zetsubou Sensei Blu ray is 720 so I assume it is the same as the dvd.

Since I'm in Japan and second hand DVDs are pretty cheap I'm thinking about buying Sayonara Zetsubou Sensei so I can get wallpapers as I please. Any recommendations of other series?