As in70 said - black is no refraction, white is full refraction. So plug the bw picture into the refract value. You can set the refract value itself to 1. Now it will not do any refraction on the black areas. No need to invert the picture

You can raise the value for the diffuse channel as well to increase the texture and contrast

But a screenshot of the material would be good to see now. That way we can see if other changes need to be made