I don't know of a tutorial, but it is rather simple. Ill explain:
After generating the terrain maps, the files are saved as .dds textures inside the levels "Textures" folder. Depending on the size of the map there will be a different amount of textures. The smallest map uses 4, which all connect to form the final product. Like a grid. If you use WinRFA to extract the level to your desktop or some other directory, you can use Photoshop to open those textures, edit them, and then save them as 16 bit .dds files to the folder and name from hence they came. Usually, you'll want to place snow or some other bright texture underneath all buildings, to highlight where they are for reference, and then in Photoshop you can do anything you want to it. Even put your face on the battlefield. The same goes for the Minimap, in the Menu folder you will find the ingamemap.dds, using Photoshop you can edit that to get different designs, although players using low texture settings in game will have a blurry map, where as high textures will have a good map.
If you want shadows, generate the shadows and then work with photoshop, otherwise you'll have to do all the PS work all over again. Photoshop can especially be used to touch up shadows, make them look nice.
Example of a Minimap:

The Original developers used photoshop for their maps, you can tell by looking at there minimaps, and terrains as well.
The thing I like the most, is you can also use Photoshop to edit the detail.dds, which is the texture which is blended on top of the others to create the detail in them. Instead of the default thing, you have freedom to edit it whichever way you want. Want rubble? Make it rubble, want grass, make it grass. Its not just this texture full of noise and random snippits of pictures here and there. I used it on a modified berlin map I did, I had no screenshots of that stage though. If you like your clan to have markings and things on the map to signify who made it, its a good Idea to photoshop it on to the minimap, so you dont actually have to do it on the map itself, and risk uglying up a nice thing.
I warn you however, Large maps will take awhile, alot of textures involved. I suggest this mainly for small maps, detail texture, and minimaps.