In his book The Zapruder Film: Reframing JFK's Assassination (University Press of Kansas, 2003), Dr. David Wrone makes a convincing case that Oswald, not Bill Lovelady, is the man seen standing in the TSBD doorway in the Altgens photo. Until very recently, I made the mistake of accepting the HSCA PEP's identification of the man as Lovelady. I should have known better. In fact, again until very recently, I always viewed the attempts to identify the man as Oswald as amateurish and embarrassing to the multiple-gunmen position.
When I recently bought Wrone's book and saw that it had a chapter on the man in the doorway (chapter 11), I assumed that Wrone identified the man as Lovelady. As I read the chapter, I soon realized that he did not, but I also saw that he made a very strong case for the Oswald identification. When I got to the last page of the chapter, I thought to myself, "Holy cow! This is powerful stuff!"
Of course, since the Altgens photo shows Oswald standing in the TSBD doorway, he could not have been up on the sixth floor firing at Kennedy.