I like the picture where they're sitting at the table, and here's why:
If the discs contain episodes from the first two seasons, then the picture on the cover should reflect that. The other three pictures are too new.
I'm not sure if the picture of them sitting was taken sometime during the first two seasons or not, but it certainly is an earlier picture than the other three. The others were taken during Season 6 or later, because Julia didn't change her hair to that style until at least after Season 5.