Look at your content boards as an opportunity to gain content/photos on an on-going basis. You don't want to run one-off competitions because you need content flowing in 365 days a year. Think of an incentive that is evergreen. Of course, you can create 12 monthly photo competitions back to back or run a competition for 3 months at a time giving 4 prizes per year. You could also consider spot and random prizes.
Shuttlerock also has a coupon system. This will mean that every time a customer uploads a photo (and shares it) they will get an instant prize. This may be a voucher for a small gift, a coupon for a discount of an existing product or for a further purchase in the future. The offer is up to the client.
For a content collection overview see - https://www.youtube.com/watch?v=pLnizfBoB2I&index=5&list=PLRNxWZk7MeRpQpOa2w_TNfMS_YBi6etwa
For more info on photo contests - https://www.youtube.com/watch?v=ADN6s2w_K-s&list=PLRNxWZk7MeRpQpOa2w_TNfMS_YBi6etwa&index=7