One of the main reasons to host a photo sharing contest is to engage your followers. If you are going to create a new product, ask them to send in images for you to promote it online. Not only are you getting free design work, you are going to make your customers feel like they are a part of the process, and like you truly want them to enjoy the new product line that you are planning on introducing to them in the future.
By engaging your followers, by keeping them in the loop and by allowing them to take part in the design or implementation phases, it will bring you and your customers closer. Even if you are not giving out a physical prize in the contest, the simple fact that you want their input and want them to help in the design phases, is going to be incentive enough for hundreds if not thousands of current (and new) followers to engage in the contest.
Although Instagram is a great place to share images, if you are planning on getting your customers and followers involved, using the power of Facebook is a great way to increase the number of participants and to ensure as many people as possible take part in your social competitions. Facebook allows you to connect to Instagram, allows more people to see the feeds and updates and allows people who are not following you on Instagram, to take part in the contest.
The end result is that you will receive more photos, your contest will garner more attention and you will eventually receive the best content to use in the end. So, use the feeds, promote, and keep your followers in the loop using Facebook to leverage your Instagram contests.