In the newest updated Version of TMNT the size and spritepositions for the Character Portraits have changed, because of that, all Skins look wrong in the Character Select screen until those have been updated by the skin creators. However the ingame sprites still work as before.
(Fixed in 1.3.0)

Let's you add skins for fighters to the game via content packs.
Skins are assets swaps and merges that are tied to a selected skin.
To switch between skins, press left or right in the character select screen.
You can download an Example-Content-Pack in the files.

Requirest the latest version of TMNT Mod Api (1.1.1+)




Install
First install TMNT Mod Api. That creates a Mods folder on first launch. In that folder you extract this mod, so that the filestructure looks like this:
-Mods
——CustomSkins
————CustomSkins.dll
————manifest.json

ContentPacks for this are installed by also extracting them into the Mods folder, like this:
-Mods
——CustomSkins
————CustomSkins.dll
————manifest.json
——GokuCaseys
————content.json
————manifest.json
————...


Latest Changes:

1.3.0 Build in automatic corrections for older textures. For this, now always add the current PackVersion to the content.json. This way the mod knows if it's old or new and which version. The current version is 1, the last would have been 0, but 0 is also the default, so older content packs don't need to add it.

1.1.0 adds audio patches, you add them to the content.json just like any other patch.
To figure out the assetnames/folderstructure for the audio you want to replace you can use: SoundExtractor
Example content.json from the April Demo (with Sound):
{
  "PackVersion": 1,
  "Skins": [
{
"Character": "April",
"Name": "April (Red)",
"MenuColor": "Red",
"Patches":[
{
"Asset":"2d\\Animations\\Players\\April\\AprilTexture",
"Patch":"AprilTexture.png",
"PatchType": "Replace" 
},
{
"Asset":"2d\\Animations\\Menu\\CharacterSelect\\CharacterSelectTexture",
"Patch":"CharacterSelectTexture.png",
"PatchType": "Merge" 
},
{
"Asset":"Audio\\VO\\April\\Selected",
"Patch":"Selected.wav"
}]
}
]
}
(This replaces what April says when the character is selected in the character-menu)


How to make a Content Pack for Custom Skins:

ContentPacks are installed like any other other mod in TMNT Mod Api, by having a folder with a manifes.json file inside the Mods folder.
The manifest file for ContentPacks needs to have these basic informations about the mod:

manifest.json

{
  "Id": "[Any.Unique.ID.You.Want]",
  "Name": "[Name for the Mod]",
  "Description": "[Description of the Mod]",
  "Author": "[Name of the Author]",
  "Version": "[Version in the format X.X.X]",
  "ContentPackFor": "Platonymous.CustomSkins"
}

This is what let's the API know that this is a ContentPack for CustomSkins:
  "ContentPackFor": "Platonymous.CustomSkins"

manifest.json From the example:
{
  "Id": "Platonymous.CustomSkinsDemo1",
  "Name": "Custom Skins Demo 1",
  "Description": "B/W Michelangelo",
  "Author": "Platonymous",
  "Version": "1.0.0",
  "ContentPackFor": "Platonymous.CustomSkins"
}


The actual information for the skins you want to add go into a seperare conten.json file.

content.json
{
  "PackVersion": 1,
  "Skins": [
{
"Character": "[Name of the Character the Skin is for]",
"Name": "[Name of the Character when the Skin is applied]",
"MenuColor": "[Menu-Color (Name)]",
"Patches":[
{
"Asset":"[Assetpath seperated by \\]",
"Patch":"[Patch file in your mods folder].png",
"PatchType": "[Replace or Merge]" 
},
... [As many Patches as you need]
]
},
... [As many Skins as you want]
]
}

Names of available Colors: http://www.foszor.com/blog/xna-color-chart/

PatchTypes
Replace: Replaced the original Texture completely with the one you provide.
Merge: Copies every pixel from your Patch-File on to it's original. Transparent pixels are ignored. Magenta-Pixel [RGB(255,0,255)] are not copied, but instead copy as transparent pixels on to the original (if you need to delete something in the original)

Both, Replace & Merge require your patch file to have the exact same pixel-sizes as the texture you want to replace or patch.

Asset
The Assetpath is the path to the file you want to patch/replace relative to the Content-Folder without the file-extension(.zxnb), with folders seperated by "\\"
To get all the Images you need to start, use TextureExtractor to turn all the zxnb images into pngs. 

You can put as many Skins and as many Patches as you want into the same conten.json file.

content.json From the example:
{
  "PackVersion": 1,
  "Skins": [
{
"Character": "Michelangelo",
"Name": "Monongelo",
"MenuColor": "DarkSlateGray",
"Patches":[
{
"Asset":"2d\\Animations\\Players\\Michelangelo\\MichelangeloTexture",
"Patch":"bwanimations.png",
"PatchType": "Replace" 
},
{
"Asset":"2d\\Animations\\Menu\\CharacterSelect\\CharacterSelectTexture",
"Patch":"bwportrait.png",
"PatchType": "Merge" 
}
]
}
]
}



Source: https://github.com/Platonymous/TMNTMods