The new steelbook release covers seem to have no real logic or thought behind them other than "lets put some evil characters on the covers but shove Yoda on one of them". A neat way to have done it would've been to have the following:

Maul - Obvious reasons
Jango Fett - Because it's Attack of the Clones, his genetic code is the start of the clones.
Dooku - He may not have much of a role in this one but it is Revenge of the Sith and it makes more sense to have a Sith on the cover.
Luke - Because it's A New Hope. Having this one stand out with the lone non-bad guy character makes more sense.
Vader - I don't think this needs explaining.
Emperor or Yoda - Emperor to fit in with the overall theme, or Yoda to fit in with the whole "Return of the Jedi" feeling.

Alternatively keep it the same up to A New Hope and put Tarkin on a New Hope because Vader already has the slipcase cover. The Emperor on Empire then Luke on Return of the Jedi to fit in with the whole temptation towards the dark side angle.

I don't know some kind of logic other than just throwing the bad guys on the cover just to be cool and throwing Yoda on a random one.