The PATH really is the important part. You can skip it, however, if you specify the absolute path. Try something like this:
/path/to/phantomjs render_image.js
…but that can get tiring. For a quick way to add it to your PATH, from the directory of the phantomjs executable, symbolically link it into /usr/local/bin:
sudo ln -s /path/to/phantomjs /usr/local/bin/
/usr/local/bin is likely on your PATH.