As a CGI program, this should be invoked through your web server. If you run it from the command line, it will either complain about unauthorized access or spew a PNG at you.
upsimage.cgi generates the graphical bars that make up the right side of the page generated by upsstats.cgi(8). These represent the current battery charge, utility voltage, and UPS load where available.
The images are in PNG format, and are created by linking to Boutell's excellent gd library.
upsstats will only talk to upsd(8) servers that have been defined in your hosts.conf(5). If it complains about "Access to that host is not authorized", check that file first.
The gd home page: http://www.boutell.com/gd/
The NUT (Network UPS Tools) home page: http://www.networkupstools.org/