This is a particular problem I have always struggled with. There are a few options:
- Make better sprites. If you don't know how, you have to learn. I have not figured this out yet.
- You can make a game which does not require complicated sprites, for example, Thomas was Alone is made entirely of rectangles, spruced up with graphics effects, shaders, lighting, particles, etctera. But it's important to note, the rectangular nature was critical to the design of Thomas was Alone, you can't make a game like that without a good reason without it being really obvious that you didn't put effort into the art.
- Don't worry about it. Just use terrible placeholder sprites. The optimist argues that it doesn't matter, a good game doesn't need good sprites to be good. The cynic says you need good art to get anybody to be interested in your project. But what I'm doing now is using placeholder sprites, and if I end up with a good game, then I'll deal with making the art nice, one way or another. (Making it myself, hiring someone, purchasing assets, for example. Most of that costs money though, money not worth spending until everything else is done).