There are probably two solutions for your problem depending on your environment:
1) you could postpone the generation and just provide the timestamp for the event.
2) if you are in the same network as your camera(s) you could enable rtsp and get the images from the life feed
2b) you could also use a third party application for that which generates images from rtsp and deletes everything older than X hours/days/... (depending on your storage). If the event occurs, you can copy the required images directly from the file system