so I am actually having the same issue and I can't solve it. I am on Win 10 x64 too. The error occurs in Xenko Game Studio as well as in VS 2015 and 2017. I tried different project templates (SimpleAudio, New Game) but got the same error everywhere.
I did set the environment variable to "C:\Program Files\Silicon Studio\Xenko\GamePackages\Xenko.126.96.36.199\" but the path in the error message remains "C:\Program Files\Silicon Studio\Xenko\Bin\Windows\x64\FreeImage.dll"
I also copied the whole "Bin"-Directory from "C:\Program Files\Silicon Studio\Xenko\GamePackages\Xenko.188.8.131.52\Bin" two levels up to "C:\Program Files\Silicon Studio\Xenko\Bin" but the error message stays the same:
Error: Exception in command SiliconStudio.Xenko.Assets.Textures.TextureConvertParameters: System.TypeInitializationException: Der Typeninitialisierer für "SiliconStudio.TextureConverter.TextureTool" hat eine Ausnahme verursacht. ---> System.InvalidOperationException: Could not load native library FreeImage.dll from path [C:\Program Files\Silicon Studio\Xenko\Bin\Windows\x64\FreeImage.dll] using CPU architecture x64.
Would be so great to use Xenko but it seems I got stuck right at the start. Any help is very apreciated.