Final answer:
For a pneumothorax, after an upright inspiration chest x-ray, the most useful additional images are a lateral decubitus or an expiration chest x-ray. For pneumonia, both posteroanterior (PA) and anteroposterior (AP) chest x-rays are beneficial in evaluating lung pathology.
Step-by-step explanation:
The appropriate imaging to take additionally for a pneumothorax, after having completed an upright inspiration chest x-ray, is a lateral decubitus position or an expiration chest x-ray. The lateral decubitus position is particularly useful to detect small pneumothoraxes that may not be visible on the standard posterior-anterior (PA) chest x-ray by allowing free air to rise to the highest point which is now the lateral chest wall. The expiration chest x-ray increases the visibility of a pneumothorax by reducing the volume of the lung, thereby potentially making a small pneumothorax more visible.
In cases where pneumonia is suspected, as in the background information provided, and a chest radiograph shows anomalies such as cavities, opacities, or consolidation, a posteroanterior (PA) chest or anteroposterior (AP) chest x-ray is useful to evaluate lung pathology further. Both positions allow for clear views of the lung fields to assess the presence of fluid or infiltrates indicative of pneumonia, with the PA view being the standard position.