See the following website for some recent research on this:

http://people.virginia.edu/~jh3v/retouchinghistory/essay.html

An excerpt:

Retouching History:
The Modern Falsification of a Civil War Photograph
Jerome S. Handler and Michael L. Tuite, Jr. [1]

Introduction

“In the past decade,” the Yale historian David Blight has recently written,
“the neo-Confederate fringe of Civil War enthusiasm . . . has contended that
thousands of African Americans, slave and free, willingly joined the
Confederate war effort as soldiers and fought for their ‘homeland’ . . . .
Slaves’ fidelity to their masters’ cause - - a falsehood constructed to
support claims that the war was not about slavery - - has long formed one of
the staple arguments in Lost Cause ideology.” [2]

In this paper we discuss a graphic example of Blight’s contention by
examining a Civil War-era posed studio photograph of black Union soldiers
with a white officer. We maintain that this photograph has been deliberately
falsified in recent years by an unknown person/s sympathetic to the
Confederacy. This falsified or fabricated photo, purporting to be of the 1st
Louisiana Native Guards (Confederate), has been taken to promote
Neo-Confederate views, to accuse Union propagandists of duplicity, and to
show that black soldiers were involved in the armed defense of the
Confederacy. . . . .