How to add additional content to an apk?

I am developing a wallpaper app and I wanted to make different categories but i can't add all of the images to the app because I don't want the app to be big, so I thought of making the user downloads the app, it will be empty with no images at all then he chooses which category he wants to have and download it (something like downloading a dlc for games but instead extra images for the app) but I don't want it to be downloaded to the phone sd card I want it to be in the app file so he won't be able to reach it without the app.

So, if you know how to download images to the app file and how to make a reference to each image after being downloaded, that would be a great help, thanks