Image Map
This is a program that lets you convert images to Minecraft maps using the latest (1.12) colors.
Click here to download the latest version:
https://github.com/tryashtar/image-map/raw/master/Image%20Map/bin/Debug/Image%20Map.exe
How to use
Click Open, then browse for any number of image files. Click Export, and browse somewhere to save all the images as maps. To put them in your Minecraft world, save them to world/data.