Memento pattern lack

Question

in my project about image processing I need to have Undo/Redo button. Briefly saying I do multiple operations on Bitmaps which then I assign to picturebox. I found this article http://www.codeproject.com/Articles/10576/An-Undo-Redo-Buffer-Framework I created my Originator based on the one from example code.

It looks like that

class Originator : ISupportMemento
{
    private Bitmap state;
    public IMemento Memento
    {
        get
        {
            Memento mcm = new Memento();
            mcm.State = GetMyState();
            return mcm;
        }
        set
        {
            SetMyState(value.State);
        }
    }

    protected object GetMyState()
    {
        return state;
    }

    protected void SetMyState(object newstate)
    {
        state = (Bitmap)newstate;
    }

}

But when i am trying to assing a bitmap object to it, Compilator is messaging an error.

    UndoBuffer bufferr = new UndoBuffer();
    Originator orig;
    orig.Memento = (object)_baseimg;

Could you please help me, very sorry for my bad english.