All the regular rules and numbers apply: 2 shots per user (3 for contributors), 1 shot per movie. Uploading will be open for a week, solving will be 10 days.
I have one question : Can I upload (for a contest) a picture from a movie with which I have already uploaded two snapshots ? (I'm not a supporter so usually I'm stuck to max two shots from the same movie, Does the rule still apply in contests ?)
Yes, you can! Vault shots are not included in that rule: you can upload a vault shot when you already have 2 in archive, but if you have 1 in archive + 1 in vault, you can still post one in NS as well (it doesn't count toward the limit).