When hosting these competitions, each hashtag that you use is going to have an RSS feed. This is what is going to make it possible for you to connect through Facebook, and through other devices which are capable of opening up an RSS feed. Whichever images the users are interested in, they will simply point their device at the RSS tag, and it will pull up that photo as well as every additional photo which has the same tag.
Using the RSS feeds is the best way for event organizers to keep track of all the images that are submitted. It will allow them to categorize the feeds on Facebook as well as on Instagram, so that users will quickly and easily be able to open an image or a specific category when they sign on.
In addition to the event organizers, anyone who wants to see the images which have been submitted are going to be able to do so simply by pointing the RSS reader device at the RSS tag, which will open the posts that are linked to that specific tag.