Sipi is a high-performance media server developed by the Digital Humanities Lab at the University of Basel. It is designed to be used by archives, libraries, and other institutions that need to preserve high-quality images while making them available online. Sipi implements the International Image Interoperability Framework (IIIF), and efficiently converts between image formats, preserving metadata contained in image files. In particular, if images are stored in JPEG 2000 format, Sipi can convert them on the fly to formats that are commonly used on the Internet. Sipi offers a flexible framework for specifying authentication and authorization logic in Lua scripts, and supports restricted access to images, either by reducing image dimensions or by adding watermarks. It can easily be integrated with Knora.