Install GIMP 3.04 or later if you haven't already. Import a terrain image, with clear biome-distinctions into GIMP. Preferably, find the original form of that image, without any JPEG distortion.

Be aware of the terrain types you want to use in the map.



1.
Make sure image is in RGB mode by going to Image (in the top toolbar) > Mode > RGB
In the bottom right, you can see layers. Right click on the image layer, and select "Remove Alpha Channel". This will get rid of transparency in the image, but sometimes this option will be grayed out due to there being no transparency.
2.
In the same menu (Image > Mode) select "Indexed".
A prompt will appear.
Select "Generate optimum palette" and set "Maximum number of colors" to the rough number of unique colors in the image. For realistic maps, i suggest doing around 10 or 12. There is only 8 terrains as of yet, but we will reduce this down to 8 in the next step.
For simple maps, try to set the number to the number of colors that you can count.












Depending on the size of the image and your computer's power, this step may take some time.

Now, we will reduce the amount of colors down to 8 or less.
1.
Select the color-picker icon on the left side of the software.
You will see the created palette here. Make sure that there is no perfect black or white colors. If there is, select the color on the palette, click on the "Edit this color" icon underneath the palette, and change it to a different color that is not perfect black/white.
Go to Image > Mode and set it back to RGB.
2.
Use the select by color tool (this icon in toolbox or go to Select > By Color) and select all colors that reflect a certain biome. In the menu on the left side, make sure "Threshold" is set to 0, Antialiasing is off, and Feather Edges is off aswell. For example, in this map, i will select the colors which i would like to be water. Remember to press SHIFT to select multiple things at once!

3.
After you select all colors that are for a certain terrain type, select the color picker icon, () and pick the proper terrain color. The following are the colors for different terrain types.
Basic Land: 000000
Forest: 1f1f1f
Hills: 333333
Desert: 666666
Tundra: 808080
Mountain: 999999
Crossing: cccccc
Water: ffffff
After you pick the color, while still selecting all appropriate pixels, go to Edit > Fill with FG Color.
Do this with every terrain type present.

Now, you should have an image that looks like an AOC map.

But, its still likely too big to run smoothly.

1.
Go to Image > Scale image
A prompt will appear.
Set width to 450 for a tiny map, 750 for a small map, 1200 for a normal map, 1700 for a large map, and 2200 or higher for a very large map.
TURN INTERPOLATION TO "NONE", OTHERWISE THE MAP WILL GET SCREWED UP!1!!111!1!!!

Go to File > Export As, and change the file extension to PNG. Set compression level to 0.


After this is completed, the map will have many functional issues. Mountains and crossings may also not be present, so you will have to add them in the in-game editor.
Make sure to check for;
Stray Pixels
Unintended terrain around coasts or in archipelagos

Here is my final result without finishing up.

If you want to turn this into a scenario, you should find a political map using the same dimensions and projection, and then use some layer-magic to overlay a slightly opaque political map with 'lock' toggled on, over the terrain map.