I am not going to praise great quality of your first pictures, at least you have plenty of room for easy easy improvements. LOL
For example you can try using iRray decals for blood and dirt instead of painting that on the texture and find some better way to do decapitations than replacing head with that red alien creature

But in the last picture here, you did everything perfect.
maybe it is juts pointless to render them in so huge resolution and also PNG. It is not like anyone even has monitor to see it.

Of course you are very welcome with your work. Do not hesitate to ask if you need some help on doing something specific.