Once you have posted the contest information, in order to elicit as many images as possible from your followers, you want to keep them in the loop.
A good way to do this is to promote using Facebook. You can inform them of the prizes that are going to be given out to winners, or let them know what the images are going to be used for once the contest is over. You want to give people incentive to take part in your contest; after all, you are going to use the images that they submit, and the creative message that is submitted along with the hashtag for your business benefit.
Since this is the case, people who take part are going to expect something in return. Regardless of how big or small it is, make sure people understand what the contest is about, what they are going to receive if they win (if anything) and every other piece of relevant information about the contest.
By promoting the contest on Facebook, you are going to elicit more people to take part in it. More people are going to view it, and more people are going to share it. If a follower likes the contest, this will automatically inform their friends on Facebook of the contest as well. In turn, more people are going to learn about the contest when you use the social media site to promote it, and you are going to receive more response and more photos for the contest when a larger audience is aware of it.
Using the social media platform is in your best interest and it does not take much effort for you to set it up.