While you're building the game, you can just use placeholder graphics from a website like http://opengameart.org http://opengameart.org. Alternatively, you can

While you're building the game, you can just use placeholder graphics from a website like http://opengameart.org http://opengameart.org. Alternatively, you can create super-simple graphics using the GIMP -- simple configurations of geometric shapes work very well -- and use them as placeholders. When you feel you are ready to incorporate "real" graphics into your game, you can hire somebody to create them for you (or adapt them from existing graphics). Also check out /r/gamedev. They will be able to help you out with this.