Nowadays, offloading storage and processing capacity to cloud servers is a growing trend. This happens because high storage capacity and powerful processors are expensive, whilst cloud services provide a cheaper, ongoing, and reliable solution. The problem with cloud-based solutions is that servers are highly accessible through the Internet and therefore considerably exposed to hackers and malware. In this paper, we design and implement Darkroom, a secure image processing service for the cloud leveraging ARM TrustZone technology. Our system enables users to securely process image data in a secure environment that prevents exposure of sensitive data to the operating system. We evaluate our system and observe that our solution adds a small overhead to image processing when compared to computer platforms that require the entire operating system to be trusted.