One of the most difficult aspects of game development can be sourcing quality graphical assets, whether 2D or 3D. Partnering with an artist is usually your best bet, but finding an artist who shares your vision and is willing to partner for a share in the final product can also be difficult.
Not to mention it takes time for graphics to be made from scratch. And sometimes an extremely long time.
For those of us who don’t need an exceptionally unique art style, there are a wealth of sites and resources for purchasing pre-made assets that look great and are yours instantly for a low price.
These are listed here. Don’t forget to add your own if you know of a graphics goldmine.