ohr-cartographer is a command-line tool for rendering maps in .rpg files as .png files, written by Spencer Zhang. Each tile appears in full size 20x20, unlike the built-in option to export map layers with one pixel per tile.
Download from GitHub here. Click on "Clone or Download" at the top right and then "Download ZIP".
Windows users should run bin\mapper.exe. GNU/Linux and Mac OSX users should instead run from the commandline (follow the instructions on GitHub). On other platforms, you will need to provide a copy of the unlump utility you compiled from source yourself.