Your first issue is that I believe you are trying to use alpha information on a .png with no alpha channel. RBG Pngs contain an additional channel, in addition to red,blue and green, that contains transparency (or alpha) information. You png is in grayscale with no transparency. What you can do with it is use it to mix materials using just the black and white information. In this screenshot shot the simple red material (top) is mixed with a slightly more complex blue material material using the black and white mask as the mix factor.
Now your “body dif” image, does have alpha, you could elimate the b&wmask entirely and just use the alpha from that to mix the two materials.
Your normal map was very weak and I replaced it in this screenshot with a stronger and similar one I had made of snake scales. The mapping node controls the size of the scales.
You say that the dark image is the diffuse color map, but I believe that it is in the spec map. Let me know if I am wrong and this is supposed to be a very dark material. Here is one method of utilizing a spec map. The colorramp controls the roughness levels from black to white (0 to 1), 0 being a prefect mirror.
