void SaveImage(Texture saveTexture, string mask)
    {
        if (saveTexture != null)
        {
            RenderTexture currentActive = new RenderTexture((int)(saveTexture.width), (int)(saveTexture.height), 0, RenderTextureFormat.ARGB32);
            Graphics.Blit(saveTexture, currentActive);
            RenderTexture restoreActive = RenderTexture.active;
            RenderTexture.active = currentActive;
            Texture2D screenShot = new Texture2D((int)(currentActive.width), (int)(currentActive.height), TextureFormat.RGB24, false);
            screenShot.ReadPixels(new Rect(0, 0, currentActive.width, currentActive.height), 0, 0);
            byte[] bytes = screenShot.EncodeToPNG();
            string filename = "D://SavePicPath//" + System.DateTime.Now.ToString("yyyy-MM-dd_HH-mm-ss") + mask + ".png";
            System.IO.File.WriteAllBytes(filename, bytes);
            Texture2D.DestroyImmediate(screenShot);
            screenShot = null;
            RenderTexture.active = restoreActive;
        }
        else
            Debug.Log("RenderTexture IS NULL!!!");

    }

public RenderTexture cameraTextures = new RenderTexture;

cameraTextures  = new RenderTexture(2560, 1280, 24, RenderTextureFormat.Default);

SaveImage(cameraTextures , "Unity");