Let's see here. In case you don't own the pictures/GIFs or what not, then you shouldn't bundle your game with them. That is pretty hard to defend in court.
What you could do, is let your users add the pictures that they like themselves to the game separately. In Twine, we are talking about URLs to images, write a simple ReadMe.txt that explains where the pictures folder needs to be. You could even make a CVS sheet and map each image to a character or action and provide that as well, if people want different pictures in a certain location. It is of course extra gruntwork to index and describe each picture slot (index, supossed picture content, size, role etc.)
This way you can steer clear from the legal BS and provide a bit of customization for your players. While other players will curse you for not giving them the complete package, which , you can hopefully live with.
Once you get a nice income out of your game, you can start commissioning art for your game and start packaging your future releases with content that you own.