In descr_aerial_map_ground_types.txt some ground_types in some climates have two tga files given next to them. The second (i.e. the rightmost one) is what shows up on the strat map for that ground type in that climate during winter. If you delete the second entry ("artic.tga"), it should just use the summer tga all year round. Of course, that only does the job on the strat map. Battles will still be snowy.