Even if I can't work in Photoshop, I can still get a couple of things done!

I've always drawn the Infernals in a kind of simplified manner, but that's just because we can't actually see their full bodies underneath their coat of flame. In actuality, these raptor-like fire creatures are a lot more like insects and arachnids!

The reason for their fiery exterior is a gas bladder, which is the green-yellow colored sac that extends along the back of their head and through their dorsal spines. Shooting the creature in the back of the head to rupture the sac (or dousing it with a hose, really, but you kind of need to make sure to soak the entire creature) is the only way to "kill" it -- as long as the creature can combust, its body can regenerate!

The creature's entire inner body is like tar. There's a dry little outer "skin" to the tar, but beneath that the Infernals are essentially made of formless black ooze. This is why they can regenerate -- if, say, their arm is blown off, it can very easily be replaced by moving the Infernal's body mass around. The Infernals' only real vital organ is their gas bladder; they continually combust the gas let out by this sac, and the fire completely coats most of their body. This fire actually contributes to the Infernals forming a "skin" around their amorphous bodies -- without the constant intense heat from their fire, the Infernals would not be able to form a proper skin and would essentially melt into a blob. Infernals can "shut off" their fire, though can only do so for short periods of time.

Again, the only way to permanently kill these things is by rupturing their gas bladder. You can see why that's a bit hard to do.