Open Source Filmmaker. At startup, this window open. Click the open session button, then in the explorer select your session (*.dmx file).
When the session is loaded, you have a chance to modify the camera to get the best angle. (camera controls: mouse + WASD)
Save your poster
Once you've edited your session, go in the menu File --> Export --> Poster. Choose your file name. Important: the default file format is TGA, you probably want to choose a more handy PNG or JPEG file format Select your image size, then click export poster.