One of the things Ton has been trying to explain to me time and time again is the concept of Gamma correction. I never got it until I read this tutorial by Alvaro Luna Bautista. He explains how to work with the Gamma settings in Yafray, how to analyse the histograms with the Gimp and how to Gamma correct your textures for better results.
To be honest, I'm still not sure I understand Gamma for 100% now, so this tutorial is probably more useful for you than for me ;-)