In this post I am going to demonstrate how to serve images from a REST endpoint for a situation when the images are not stored as static files, but generated on the fly given some original form. This use case will be described with application of Flask, OpenCV and NumPy. A situation when this functionality is needed is when the original images are too large, and their downscaled representations are sometimes required, or when the original format of the images is too heavy to be served via a web service (like BMP).
When it comes to building and installing OpenCV with Python support on *nix platforms, the collection of tutorials by Adrian Rosebrock is the best. He provides detailed description of the required steps, as well as motivation for better development practices. In particular, Adrian creates a dedicated virtualenv environment, installs NumPy in it, and builds the whole thing having this environment activated. Such a solution worked pretty well for me, both on Linux and macOS.