For future people interested in how this works, I'll write how I got it working...
- 1. Download and extract the Umodel into a folder on your desktop
- 2. Create a file called fortnite.bat
- 3. Edit fortnite.bat with a text editor
umodel.exe -path="C:/Program Files/Epic Games/Fortnite" -out=\ExportOutput -export */UI/Foundation/Textures/Icons/*
- 4. Now open the Command Prompt (CMD on Windows), and navigate to the fortnite.bat (using "cd C:/path/to/fortnite.bat")
- 5. In the Command Prompt, now run "fortnite.bat"
You will be prompted for an AES key:
0x9D8C9A4A4FA082F213EED604B6E756237181685EEDA82216437617D7AA5231AFThis key is changed every new update. How to find the new one? Google 
Game Version as of date:
Unreal Engine 4.19This will save all the Images from the Fortnite game into the ExportOutput folder in the UModel folder on your desktop.
These files are now in a .TGA format, and they include an alpha channel. To convert these TGA files into transparent PNG files I used ImageMagick with PHP.
If you're using a linux DEB server with
PHP7, you may
install ImageMagick easily by running this command on your server:
sudo apt-get install php-imagick
<?php
// create new imagick object from image.jpg
$im = new Imagick( "T-Constructor-HID-Constructor-009-SR-T01-L.tga" );
// change format to png
$im->setImageFormat( "png" );
// output the image to the browser as a png
header( "Content-Type: image/png" );
echo $im;
Voila~ Now you got a transparent background PNG image from the TGA files exported from the Fortnite Game engine!