If you want to promote the contest on Facebook, the easiest way to do so is to come up with a creative hashtag, and one that is going to distinguish the competition. Not only will allow Facebook followers have the ability to see the images, see the competition and learn about the details, but they will also be able to post their own images through Facebook or through Instagram when they use the right hashtag with their image post.
By doing this, not only will a competition gain more followers and more interest, it is also going to elicit more people to want to engage in the contest and to post their own submissions, which are going to help you as the business owner in the end.
Therefore, integrating Facebook in to the mix, and using the appropriate hashtags, is a must if you want your contest to have as many people as possible taking place in it and as many images as possible being submitted to the contest.