Not unless your free trial expired before you subscribed to Pro.
If you upgraded while still on a trial, your photos are stored in their original resolution and you will not need to re-sync them after subscribing to Shoebox Pro.
If you let your free trial expire you will have to run the photo backup again to sync your original resolution photos. It's simple - after subscribing to Shoebox Pro:
Keywords: Do I Loose, Original Resolution, Trial, Syncing Photos, Pro Plan, Free Plan