osmpng is a small tool to create larger png files based on OpenStreetMap "tiles".
In principle this program downloads segments ("tiles") from OSM servers and merges them together to a single file.
The tool is hosted on GitHub.
osmpng has been written in C++ using libcurl and libpng++. Manual and build procedures are found on GitHub.
An example map of Innsbruck (~50MB) is available online.