Good question.
The image and log folders are just consuming diskspace so as long as you are not short on diskspace they will not do any harm, the images are just for the reports so they unrelated to either the database or the logs. But neither will it do any harm to delete them it just means your reports will not have images and you will not have any historical logs. Maybe I should add an option with SongKong to purge these files, or in the case of images not create them in the first place but I do what to try and keep SongKong simple.
The database does have a purpose, you can empty it even if not corrupt from within SongKong using the Empty Database task but this willl have some effect on future matching:
The SongKong database contains:
Details of every file loaded into SongKong
Details of changes made to songs by SongKong
Cached copies of releases and artists downloaded from MusicBrainz and Discogs
From time to time you may want to clear out the database, reasons could be:
To reclaim hard drive space
To force SongKong to retrieve new versions of data from MusicBrainz and Discogs
To delete changes history, so future undo changes will only undo up to this point.