How can I create thumbnails of textures loaded from compressed file?

Here is my problem,

 1. At run time I load textures from compressed file, it is compressed rar file.
 2. I create Texture2D object using Texture2D.LoadImage method.
 3. I can't resize this texture, Texture2D.Resize method doesn't work (of course I'm using Apply method) - blank texture is displayed.

So how can I create those thumbnails?

I will greatly appreciate your answers.

Add the thumbnails as separate images in your compressed archive.

edit: without resizing, does the texture appear blank too?
Maybe the format of your image is not ARGB32 (default), so you need to give it explicitly to the Texture2D constructor:

// for example
Texture2D texture = new Texture2D(512,512, TextureFormat.RGB24);