Brand names. Fighting-fit cowboys. Tawny men with cheeks you could rest highballs on. On the walls of the Walker Art Center, the images in "Richard Prince: Spiritual America" might have an even more profound impact today than they did when they were created during the stagflation generation. After all, his images of 1970s and '80s ads (rephotographs, he calls them) have been appropriated; they are faces and symbols that have taken on a life of their own as context (and the lack of it...