Ensure consistent image labeling between daemon starts

[khanu263]

Currently, image numbering is handled within the code, meaning that if the daemon is restarted, numbering is also restarted. This should instead be maintained between daemon restarts. This could probably be done intelligently by reading the image directories at startup, but it might be simpler just to have a file in each directory indicating the current image number.

Also, images should probably have some sort of timestamp in the filename.