The textures are .jpg images, and it's possible to save .jpg images without further compression. Just open the .jpg in your graphic program, do whatever changes you want, then when you save the .jpg make sure that in the options area you reduce the compression to zero or 1 and the file won't be recompressed and will maintain it's image quality.

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi