Not sure if your using Photoshop or Gimp but one thing that I'm seeing is that a majority of your renders don't match the surroundings of the sigs resulting in unequal balance. Throw in an adjustment layer on top a sig when your done and see how you like the result.
-Used hue and saturation to bump up the reds and I added a blur.
-added a blue and black gradient map and made a small vignette.
- Used a a simple soft black brush to darken the sig and added a gradient map.
I'm not saying you have to do this but I thought it was a good tip.