I believe the painting you are looking for is Across the Continent: Westward
the Course of the Empire Takes its Way - 1864 by Emmanuel Gottlieb Luetze.
The study for it hangs in the National Museum of American Art, Smithsonian
Institution. The original is a mural in the U.S. Capitol. It is a wonderful
painting, and an excellent example of the concept of Manifest Destiny.
Another excellent example is Samuel F.B. Morse's The Old House of
Repesentatives 1822 in the Corcoran Gallery of Art in Washington, D.C. You
might also look at Albert Bierstadt's works too. A wonderful book that
encompasses much of the art of the West, and consequently the concepts of
Manifest Destiny is The West of the Imagination by William H. Goetzmann and
William N. Goetzmann Isbn:0-393-02370-2. It is a wonderful book and I have
used it for reference several times in addressing Manifest Destiny. Another
stirring example of Manifest Destiny is The Last of the Buffalo by Bierstadt.
Get the kids look into the history of the buffalo and Indians, along with
who were Bierstadt's major supporters and it all gets quite complicated. I
did a paper on this painting when I was an undergrad, and fell in love with
Western art in the process. Hope this is helpful. Susan Sanders