Here is my solution with a custom function:

/// <summary>
    /// Get product image from IProductContent Model
    /// </summary>
    /// <param name="iProductContent">This model is provided by view file when it call to view product image</param>
    /// <example>_ProductBox.cshtml</example>
    /// <returns></returns>
    public static System.Collections.Generic.List<string> GetProductImageFromModel(Merchello.Web.Models.VirtualContent.IProductContent iProductContent)
    {
        System.Collections.Generic.List<string> images = new System.Collections.Generic.List<string>();
        var properties = iProductContent.Properties;
        foreach (var pro in properties)
        {
            if (pro.PropertyTypeAlias == "image")
            {
                var id = pro.DataValue;
                var umbHelper = new Umbraco.Web.UmbracoHelper(Umbraco.Web.UmbracoContext.Current);
                var content = umbHelper.Media(id);
                images.Add(content.Url);
            }
        }
        return images;
    }

Somehow Model.GetPropertyValue<List<IPublishedContent>> doesn't work. So I have to look into its property value and try to take image Id to find image url.