Add watermark feature to existing project

I bought a script on envato codecanyon, which serves as a web application that allows users to auto post to Instagram, schedule and manage Instagram accounts at the same time. There are multiple users and each user can add one or more Instagram accounts to manage. The script is completely ready and written on PHP but I need to implement a feature it doesn't have currently. I need a watermark feature. Each user will have the ability to upload a watermark and that watermark can be used automatically on scheduled images/videos. The users must have the option to turn the watermark on/off and of course change it if they wish.