I think this may just be a case of a gun that was mostly retired from the field before a photograph was able to be taken. Not that I’m saying there isn’t one, but it’s possible there isn’t. The large majority of photos are Union guns, or guns captured by the Union.
From memory, I don’t think any 24 pdr Howitzers served in the field on the Union side, except perhaps at a fort for defense. The Confederates just didn’t have many 24 pdr Howitzers, nor many Artillery photos taken overall during the war years, and that lowers the probability of finding good photos quite a bit. A lot of those guns were melted down and made into Napoleon Guns too, so that lowers the available population…
You might find a photo of the barrel alone stacked at an Arsenal during the war, but then it becomes more challenging to exactly ID.