Not much use in me posting it as a mod. It would be much easier for anyone who wants it to simply download irfanview or some other image software, and do a batch operation on the map tiles. To zip them all up would be a gig of files, approx.
Here's how I did it (extremely simple):
1) download and installed irfanview (free)
2) make a backup copy of the default map images folder which are at
World War One Gold\Data\Graph\Map
3) open up irfanview in batch mode
4) using the irfanview batch mode advanced options, change and save all the map tiles (but not the ones that start with the name 'winter') in the above folder. All I did was increase the 'gamma' of each map tile image to 2.50. You can choose any level of gamma you want.
Just make sure the modified images are in the above folder, and when next you start WWI Gold you'll see a lighter map.
If there was a way to use something other than .bmp files, it would sure make loading of the map faster, I think.