The biggest thing I see is the teeth. They should be angled out away from the face not towards the face( like in the reference you provided.) It makes it more realistic and gives it a better way to close their mouth or to eat something. You should also draw some of the insides of the mouth like the tongue and the back if its thought.

Draw a dotted line along the spine and use that as a reference for the spikes. Once you have them where you like it, draw a triangle with the dash on the back as the base. You can play around with the angles from here to get what feels right, and then add the usual tricks to simulate depth.

Here's and example of using a centerline to find the placement of the spines. Gotta find the center of your critter.

http://i54.tinypic.com/15po4fr.jpg

A central line is crucial. Remember that The Darkness is a 3D being and very much round. Therefore, if you're having difficulty drawing the body so that it doesn't look 'flat', might I suggest drawing circles along the body to give you an idea of how 3D it is? Sort of like making a bunch of curved 'cylinders', like so, except repeatedly, and not making them straight. Some circles will be much more oval than others depending on the angle of the body. Once you have that done, finding the central line (the spine, so to speak) will be much easier, and you'll be able to tell where the body will have to be moved because it's occupying the same 'space' as the spikes that you'll eventually be adding. Don't be afraid of drawing the parts of the body that are hidden behind the other parts to get a better continuous line; you can erase the parts that show through the parts in the forefront later.

I also found two more pictures that might help you with the spikes: this and this. Hope that helps some!