How to make 12 bit color images (in hexadecimal) for this screen

[Mixslav]

So I have been using this project for a while and I do not understand how did you make these hex images for the LM15SGFNZ07 screen. I have been trying all picture converters to hex but this library uses 12 bit colors (ex. 0xfff) but all I find is standard 4bit (0xff), 8bit (0xffff), 16 bit (0xffffffff) ... image converters. I just want to know where did you draw and convert pictures in hex for this project?