Most programmers can't produce large quantities of high-quality pixel art. So for a game that uses a scrolling map, it is wise to enlist artists. Ordinarily, it goes like this:
But in order to make the demo, you'll need some art assets to put in your demo. Some people prefer to rip graphics from existing non-free NES games, but it could prove disastrous if enough of these tiles managed to make it into the final release. So it's better to use assets that require nothing more than crediting the author, like these: