So what is this?

First of all, yeah, this is an ADDON, you do require the original mod's SCRIPT to even get this to work.
You can also use the CONTENT too, since that mod is configurable, it allows you to select which textures you want to use for each alien leader level.
Unassigned levels will use the previously stated level, meaning that if you have an alien species with a skin assigned on leader level 1, but none on 2 nor 3, and another, different skin in level 4, levels 2 and 3 will use 1's assigned skin.

TL;DR this mod brings more options to use that mod with, in the shape of my awful taste in colours and combinations. Be warned that they're amateur edits, mostly consisting of shifting the aliens' colours around, rather than me drawing patterns, cause I can't draw for sh-

• Download the ones you want (they're completely independent from each other).
  

Each texture download comes with an .upk and a .tga file. Unless you plan to inject the .tga file into a bigger .upk for your own sake or use it in another project, you DON'T NEED IT.
The .upk already has the .tga file inside, I decided to include the texture outside of the .upk too in case anyone wanted to use them for whatever without needing to extract the .upk file.
All I ask is that if you end up re-distributing a project that includes my skins at least try to credit me for 'em.

• Place the .upk files somewhere INSIDE XCom-Enemy-Unknown\XEW\XComGame\CookedPCConsole
  

Preferably inside a subfolder where you also have Smind's CONTENT side of the mod (if you're using it), or something like that where you can easily pinpoint the files.

• Open DefaultCharacterSkins.ini (this comes with Szmind's SCRIPT) and edit an entry for the corresponding alien you wanna add a texture for.
  

If you wanna add, say, a Muton texture, then go to the Muton section of the file. You can only have up to 10 skins assigned per species, since there's only 10 levels, going from 0 to 9 (9 being the absolute leaders like Mongo, Sargon, etc).

The lines follow a somewhat similar "logic" to Voicepacks. They follow this pattern:

AlienSkins=(iChar=NUMBER OF ALIEN,iLeaderLvl=LEVEL OF ALIEN LEADER,Skin="NAME OF .UPK FILE.NAME OF TEXTURE")

• NUMBER OF ALIEN corresponds to the alien species. Use the .ini as a guide to know each species' number.
• LEVEL OF ALIEN LEADER corresponds to the leader level that must be met for the skin to be used (without switching them in-game with CTRL + L).
  

lvl = 0 is literally an alien without any upgrades, basically the very first variations that spawn at the start of the game without bonus stats nor perks.
lvl = 5 is when they're pretty beefed up and have several upgrades.
lvl = 9 is when they're pretty much maxed up, rarely show up (they're around for certain missions like Alien Bases) and have funky names like Archon, Bashar, Atlas Drone, so on.
Read the "So what is this?" bit of this mod to know what happens if you don't assign a skin to a lvl.

• NAME OF .UPK FILE corresponds to the name of the .upk file containing the texture in question.
• NAME OF TEXTURE corresponds to the name of the texture itself. Another reason why I'm including the .tga files alongside the .upk, is because the name of the texture is literally that of the .tga file.