Some time ago (about 2 years ago) I also was trying to make NosTale "cooler".
First I had to use NosUnpack to look into NOS files:

Then I use NosPack to repack chosen spirites:

Next I use NiImage to edit spirites:

Finally I had to pack edited spirites back into nos file by NosPack.
I found old screenshots (I can't post images so I attached them). There are custom player icon and exploding mandrake?(I don't know english name) with blood. I also changed first mob into poo, but I lost files and videos :/
This OnexExplorer looks great!