Add the script to Simulation Flow (Ctrl/Cmd + F2) > SimulationPost.
Once a simulation has been reached the last simulation frame, RealFlow goes through the simulation, takes screenshots from every frame, and saves them. After this process, a message is being printed in orange to point you to the images' location.
lastFrame = scene.getMaxFrames()
if (scene.getCurrentFrame() == lastFrame):
path = scene.getExportResourcePath(2)
for frame in range(scene.getMinFrame(), lastFrame + 1):
scene.setCurrentFrame(frame)
openGLImage = scene.getCurrentViewportImage()
openGLImage.save(path)
scene.message("<font color=orange> <font size=3>The images can be found under "+path+"</font>")