You don’t need a ‘special’ hashtag, but you do should use a unique one.
Remember that anytime anyone anywhere (in the world) uses the hashtag, the photo will print at the event.
So, instead of using something like: #ourevent you should try something like #liveshotukeventparty
You can always see if a hashtag is being used (or at all). And if you are not sure, ask us and we will help you.