Update some MR map previews.

Updates the map previews of all the Best for MP maps & Empire

This removes the fancy mainland biome switch because it was the only map
to do that and we lack the tools to automate it.

Differential Revision: https://code.wildfiregames.com/D4159
This was SVN commit r25789.

binaries/data/mods/public/maps/random/gulf_of_bothnia.json

@@ -3,7 +3,7 @@
 		"Name" : "Gulf of Bothnia",
 		"Script" : "gulf_of_bothnia.js",
 		"Description" : "Players start around a gulf dotted with small islands.\n\nThe Gulf of Bothnia is the northernmost arm of the Baltic Sea.",
-		"Preview" : "gulf_of_bothnia_frozen_lake.png",
+		"Preview" : "gulf_of_bothnia_winter.png",
 		"Keywords": ["multiplayer"],
 		"CircularMap" : true,
 		"SupportedBiomes": "gulf_of_bothnia/"

binaries/data/mods/public/maps/random/mainland.json

@@ -3,7 +3,7 @@
 		"Name" : "Mainland",
 		"Script" : "mainland.js",
 		"Description" : "A typical map without any water.",
-		"Preview" : "mainland_temperate.png",
+		"Preview" : "mainland.png",
 		"Keywords": ["multiplayer"],
 		"SupportedBiomes": "generic/",
 		"CircularMap" : true

binaries/data/mods/public/maps/random/persian_highlands.json

@@ -4,7 +4,7 @@
 		"Script" : "persian_highlands.js",
 		"Description" : "A dry central plateau rich in minerals surrounded by rocky hills\n\nThe southern parts of Zagros Mountains were the heart of the Persian empires and population. Although the altitude is high, the southern parts are drier that the northern Zagros, leading to a semi-arid climate. Still there are some sparse oak forests in the higher grounds.",
 		"Keywords": ["multiplayer"],
-		"Preview" : "persian_highlands.png",
+		"Preview" : "persian_highlands_summer.png",
 		"CircularMap" : true,
 		"SupportedBiomes": "persian_highlands/"
 	}