If the DVDs are individually packaged, you can and should have individual cover scans. If they are all packaged together there is only one source for the cover scans of that particular profile.

Locally, you can do what you like, for example borrow scans from other DVDs in different releases or localities.

Quoting force:

Quote:
...6 identical pictures next to each other. ... Is this normal ?

In this case, it is.

Quote:
Why not a different picture/poster for each movie ?

Think about it: exactly which other poster would you assign to part 1, and why this and not another? 

That would prove too much of a problem for the online-db, but as Staid S Barr and awilbohk already pointed out, locally you can do whatever you want.
Find poster or cover of other releases and insert them, or if you want to keep the Box-Set-Cover and it is just the distinguishing that bothers you, fire up your grafik-program and do something like this

cya, Mithi