Pinterest is an excellent tool for promoting your business. It also provides you with an excellent environment to host a contest. Traditionally, contests have always prompted a deeper engagement with both consumers and or prospects. However, Pinterest does not provide a "free-for-all environment" for contests - these are the guidelines for hosting a contest on Pinterest.
Contest guidelines over Pinterest

• Pinterest is a visual platform which lets users discover quality content which inspire them – always give prominence to quality pinning over quantity pinning
• Make it easy for people to get involved in the contest by providing clear and simple instructions.
• You can't suggest that Pinterest in any way sponsors or endorses your contest.
• You don't have to limit people to pin from a selection — give them the freedom to pin their own ideas.
• You can't ask people to pin or repin your contest rules.
• You can't run a sweepstakes where each pin or repin, and board and like or follow equals an entry.
• Never encourage spammy behavior.
• You can't ask pinners to vote using pins, repins, boards, or likes.
• Never overdo a contest — remember even contests come with a shelf life.
• You don't need to specify on a "minimum" number of pins - one pin is all it takes.
• Re-pin your clients pins

If you are using Pinterest as part of a contest or sweepstakes, it is mandatory that you comply with all legal requirements such as - writing the official rules; clearly mentioning terms and eligibility requirements (age and residency restrictions); and adhering to marketing regulations (registration requirements and regulatory approvals).