Is async image loading possible?

You're on the right track in the comments above when you ask whether the WWW class could help. This is how I've solved the problem in the past for asynchronously loading local image files as Texture2D data:

Texture2D _content;
Material _material;

// Call this with StartCoroutine(LoadTexture(path));
IEnumerator LoadTexture(string path)
{
    // Start the asynchronous load.
    var www = new WWW("file://" + path);

    // Let the game continue running until the texture has been loaded.
    yield return www;

    // Replace current content with loaded texture.
    if (_content != null)
        Destroy(_content);
    _content = www.texture;

    // Instantiate a copy of the material, 
    // and hang onto it so we can clean up when finished with it
    if (_material == null)
        _material = GetComponent<Renderer>().material;

    _material.mainTexture = www.texture;
}

void OnDestroy()
{
    // Clean up created instances
    // (these aren't immediately garbage-collected otherwise)
    if(_material != null)
         Destroy(_material);
    if(_content != null)
         Destroy(_content);
}

I don't know how much load-balancing Unity does on its own, but it might be good to avoid kicking off all 6GB of load requests at once. ;) Try pacing them out, so you only have a few requests running at a time, to avoid any major hitches.