The pair lighting are on the left of the painting. There's another soldier behind them, and one on the right. I suspect you're right that you're looking at a cropped version. (I included the link just for electronic reference.) The full painting can be seen in the Time/Life "On to Richmond," which is where I was looking at it while writing. Paintings and artwork are touchy things: by their nature, they're already secondary sources. But they can show us things that a camera was not technologically in place to show yet, and also convey some of the attitude and mood of the times. I've always had a fondness for the sketches of Edwin Forbes, too. Even as a kid I thought he captured the "feel" of his subject.
Pine River Boys, Co I, 7th Wisconsin
"We're... Christians, what read the Bible and foller what it says about lovin' your enemies and carin' for them what despitefully use you -- that is, after you've downed 'em good and hard."
-Si Klegg and His Pard Shorty