These are the basic instructions for adding custom icons to your game.

To add Custom Icons to your Scenarios, draw them in your program of choice and save them as a 50x50 .png

Then copy them to the Scenarios folder located inside your game folder.

Load up the game and the sprite should be appear in the "External" container of the Pick Sprite Window.

The Pick Sprite Window has 3 areas, Default, Scenario and External.
Default are the Sprites that come with the game by default.
Scenario are the current custom sprites loaded for this Scenario
External are all the Sprites in the Scenarios folder

When you select an External sprite, the sprite will be given a unique identifier and its file will be copied to the Scenario folder. If you choose the same External sprite twice, it will be copied twice, so if the sprite you wish to select is already on the Scenario container, pick that one instead of importing a new one.

And that's it! Go ahead and create your Scenarios with Custom Icons and upload them to the Workshop, I look forward to playing them!

Cheers!