Single trees can be rearranged to look like a HoMM2 forest.
Here comes the first impression (some of which could be used as ressources), I reduced the size of your converted picture to 66% and then transormed it into tiles. We need some for the airports, irrigation, farmland and fortress, too. But instead try to form a more interesting landscape by using buildings or whatever you find). I wouldn't place every single ressource all over the map, that looks awkward. Not to mention those for mining (which I would say would be enough to use a simple mine pic with all kind of HOMM ressources around it - there's one in the file uploaded). That leaves us 20 little pics for ressources plus the one for the grass, totaling 21 extra graphics. And there are no ressources on grasslands, it only uses the shield tile to indicate a resource there (well a green shield in the original version, clover in mine). There are more terrains in Civ2 than in HoMM2: desert, prairie, grass, forest, hills, mountains, tundra, artic, swamp, jungle, ocean - 11, so you could reuse terrains with completely new resources or have different-looking forests or mountains. It doesn't have to be ressources only, could be nice buildings too, a volcano for example, just like in the uploaded picture.