DVD spec for the Jacket Picture is 720 x 480 for the Large and 176x112 for the Medium. The Large is shown when the disc is stopped and the Medium is used in the explorer menu. There is also a Small in spec. They are very small mpeg files.

Not all DVD authorers bother to put them on the disc, silmilarly not all bother to put the DVD text either.

As to being blurry, I've seen some really nice jackets and some really bad, depends on the guy who made them.