Well, if you want to make your image look like it is from the Sin City art, then check this out.
Get a picture of something. I took a picture of the Los Santos skyline at night:
Then desaturate the Image:
you will see that all the colour has been drained from the image. That serves our purpose as the movie was black and white
Add a Gaussian blur, value of 2:
then add the Smudge Stick effect:
now the final step. Add a Gaussian Blur, value of 0,5
If you want, you can add a tint by using the paintbrush with a very low opacity value. Look at mine (green tint):