Here's why:
* Portraits are a common feature of Hogwarts: Almost every room in Hogwarts, including the common rooms, hallways, and even the Great Hall, has portraits.
* The Headmaster's office is a place of importance: It's the office of the most powerful wizard at Hogwarts, and it would be fitting for it to be decorated with portraits of past Headmasters and other important figures.
* The Headmaster needs a connection to the past: Portraits allow the Headmaster to consult with past Headmasters and other figures for advice, a helpful tool for navigating the complex world of Hogwarts.
While there's no direct confirmation, the evidence strongly suggests that the Hogwarts office likely contains at least a few portraits.