Model Containing List of Models (MVC-3, Razor)

This problem has been plaguing me for two days now. There are some similar posts, but none that address my problem completely.

Using MVC-3, Razor syntax:

-- EDIT.cshtml --

@using (Html.BeginForm("Edit", "My", FormMethod.Post, new { enctype = "multipart/form-data" }))
{
    <!-- Some fields... -->
    <div class="editor-field">
        @Html.TextAreaFor(m => m.LongDescription)
        @Html.ValidationMessageFor(m => m.LongDescription)
    </div>

    <!-- Some more fields work... Including picture upload (summary).-->
    <input name="button" type="submit" value="Add Picture" />

    <!-- Picture Item display -->
    @foreach(var thumbnail in Model.ThumbnailImagePathAndNames) 
    {
      <img src="@Url.Content(@thumbnail.ThumbnailPicturePath)" alt="" width="200" />
      @Html.RadioButtonFor(o=>o.SelectedImage, @thumbnail.ImageGUID)  Primary Picture 
      <!-- Checkbox to mark for deletion -->
      @Html.CheckBoxFor(o=>thumbnail.Delete) Delete ???????? <!---- Here is a problem - I don't understand how this should work -->
    }
    <input id="Submit1" name="button" type="submit" value="Complete Edit!" />
}

-- MyController.cs --

 [HttpPost]
 public ActionResult Edit(String button, HttpPostedFileBase file, MyMainModel model)
 {
     // if button = submit picture,  work with picture here and break(long story)

     // save model data
         // if valid, save and redirect


     // not valid or error, load up view like normal but with error messages
     model.LoadThumbnails();
     return View(model);

 }

-- MyMainModel.cs --

public class MyMainModel
{
    // some properties...
     public Guid? SelectedImage { get; set; }

    [Display(Name = "Detailed Description")]
    public String LongDescription { get; set; }

    // some more properties....


    // and finally my list of models