Considering you're making a tile based game, you should have an editor for creating your world. So, the editor needs to be able to load the tiles, and give them unique ID's. The ID's could just be numbers, with 1 being the 1st tile it loads, but the ID's need to match the tile persistently (ie, it can't change), so it should be saved in a file somewhere (hopefully with a know format, like JSON). Then, when your editor creates your world, it just uses the Tile ID for the tile reference. When your game loads the map, it will look at the tile ID, then look into the tile Id file, and it will know which tile image to load based on the Tile ID.