The painting is wonderful, especially with regards to the two artillery pieces depicted. Griffin’s Battery was Battery D, 5th US Artillery, aka “The West Point Battery.” It consisted of four 10-pound Parrotts and two 12-pound howitzers.
The full battery was stationed to the left of the Henry House, but Griffin was ordered to move a section of two guns to the right of Henry House. This is what is shown in the painting. The section was deployed without sufficient infantry support, and the crews were decimated by Confederate sharpshooters. This occurred prior to the events depicted in the painting.
Per the painting, the two guns deployed must have been the 12-pound howitzers.