All API methods are declared "public", which makes them unusable. Please use "private" instead.

  private void EnsureInitialized()
    {
        int num = Mathf.Max((int)this.paintableSources.Length, 1);
        if (this.textureIDs == null || (int)this.textureIDs.Length != num)
        {
            Array.Resize<uint>(ref this.textureIDs, num);
        }
    }

Furthermore, consider adding the "EnsureInitialized" method into the Image download whenever the textureIDs array is null. Otherwise NREs will arise. Also gets called in the native UpdateSign method. Thanks!

You can use public methods if using the // Requires magic comment, otherwise yes, they should be private.