Image HELL

So I am currently developing an android app, and it is a sort of "picture library" you can say. It's going to contain many images and I am afraid that I will deter users from downloading if the size of the app is over 200mb! I was wondering how you guys would do it. Now, the app has to work offline so all the images have to come with the app.

What I was thinking was giving them the app without the images and then inside the app there would be a download button where they can download all the images, but still that would take up a lot of room. Any sugguestions?