It seems adopting high tree-perched firing positions could serve two purposes - observational and better targeting.
If the accuracy of Forbes's sketch is accepted, then cannot imagine how these shown 'tree frogs' achieved enough stability in their assumed firing positions to be able to reload their weapons, while perched.
It's noted in the sketch that two of these tree-mounted riflemen greatly risked being spotted, by exposure in the open without using the protection of being behind the tree trunk. Could it be that this pair of shooters were more enthusiastic infantrymen, rather than experienced highly skilled sharpshooters with specialized training.