Hi all,
this is my first step by step tutorial on how to add your own beautifull goody hut images to the campaign map.
This tutorial NEEDS to be refined about many details.
1- Download a .dds reader or saver, like paint.net, that is great, free, light, and just does the job :
http://www.clubic.com/lancer-le-telechargement-26919-0-paint-net.html
sorry the site is french, you could also google it, but I don't give the link, it could lead to unsafe place, I know the french site is safe.
2- Create a "MyMod" folder in the Elemental/Gfx folder. I guess it could go elsewhere, but I don't think it will hurt for now (needs to be refined).
3- Download and save a great picture (I use google pictures)
4- Save the picture in a .dds format, using Paint.net or whatever. Save it in the MyMod folder, and name it "newpicture", for example.
5- In your Mymod_goodyhut file, in the "DrawnIcon" line, indicate the path to your saved picture, and its name, under the "TileDesign", just like that:
<TileDesign>FRONT_AMAZING_TREE</TileDesign>
<DrawnIcon>Gfx/MyMod/newpicture.dds</DrawnIcon>
<IconSize>140</IconSize>
Adjust the icon size, I put mine to 140, but you can put any value it seems (needs to be refined).
6- Et voilà, load the game, launch it, and you should see your great new campaign map goody hut icon 
The icon size and overall quality of the icon (transparency) needs to be refined.
Hope it helps, feel free to add some precisions, or ask if you have any questions 