Well, I am no pro, but working with the WEAPONS, all I did was export the files from the Ark Dev Kit, open them in Blender, exported the textures from blender, worked on them in GIMP, reimported to blender, attached the textures and reimport to the dev kit for use. I have been able to make quite a few changes just doing this. The dino’s I am sure can’t be that far off.
Essentially, at the very least, you should be able to directly search and export the TGA files for the dino textures, edit them in Gimp of some other program, and reimport as a TGA file for use. From there, you simply go to the dino model file, find the part where it lists the OLD texture for use on the model, and tell it to use YOUR NEW TGA file instead. Should be that easy.
Importing TO/From Blender
This video shows how to improt/export from dev kit to blender, SAME EXACT PROCESS IN YOUR CASE, except you want to find the TGA file associated with the texture for the specific dino, export THAT for use in GIMP, export from GIMP back to a TGA file, and reimport that into ARK Dev kit for use on the dinos! I will make a quick video on how to do this, because, your right, lots of words here for what is actually a simple process, easy to do! Will post in a couple of minutes!