I'm using texture material on the brush and the quality is way worse than I would like.Original 
Painted with a brush

I'm using texture material on the brush and the quality is way worse than I would like.Original 
Painted with a brush

When creating your texture that you are going to paint to, increase this number. (smaller is lower resolution, larger will allow for more detail) It is a good idea to keep powers of 2, such as 1024, 2048, 4096, 8192 Etc.

In your case, when you already have an image made, you can adjust the size of your image in the tools panel of the UV/Image Editor
