Theory!
The FBI told Mr Lovelady to bring a short-sleeved shirt to the photograph session, in order that his appearance in those photos would conform to that of the man in the doorway!
But!
Mr Lovelady's arm in the Altgens photo never
was Mr Lovelady's arm, and the realization of this led to...
complications.
But if not Mr Lovelady's arm, then what?
Let's start with this!:

Now!
What if...

To illustrate more graphically!:
Another person on those steps (lower than Mr Lovelady, not as far west as Mr Carl Edward Jones)... shielding their eyes from the sun. 