One way could be to create a image with the size of your map. and each pixel corresponds to a tile.
and from there you in code chek each color and set a specific tile in the game. so that a green color could be grass, and red could be lava.
You can break this down in more stuff, like a colision image aswell, setting 0,0,0 (black) = no collision, and any thing else is collision.