I found a really excellent article to read on civil war art here I found the information about the illustrators that worked for the press very informative and interesting to read. The article is too long to copy and paste here, so if you enjoy reading about this history please check out the link.
One of the names mentioned in this article was William Waud "is particularly noted for his accuracy, and Harper's acclaimed him in 1865 as 'the most important artist-correspondent of the Civil War'"
Information about William and his brother Alfred here Alfred Waud was also an illustrator during the civil war.
some of William Waud's works:
These photos were found with the library of congress here mixed in with his brother Alfred's works among others