The erect chest x-ray shows a crescent of air under the left hemidiaphragm, suggesting the presence of pneumoperitoneum. This patient had a perforated peptic ulcer.

To determine the presence of a pneumoperitoneum, always look for air under the left hemidiaphragm too on an erect chest x-ray.

How do you differentiate a gastric bubble from a pneumoperitoneum?

A gastric bubble has a thick stomach lining separating it from the left lung, whereas a pneumoperitoneum has only a thin membrane of the diaphragm separating it from the left lung.

Further reading here.