How do i make large tilemaps that i can individually select and edit each tile?

Hello, I'm trying to make a mobile app where you can organize your photos. Imagine a large tilemap (1000x1000) where you can select an individual tile by tapping on it and are able to set the texture to a photo from your phones camera roll.

So my question is, how do i make a large tilemap and have each one of the tiles be editable in game?

Any help would be appreciated.