Any of the three folders should be fine really. (Images, IconSets or User Data.) Can't see any real reason any would be a problem, so whichever you like best.
If you put things in one of the other dirs, it's worth remembering that not all config dirs are included in exports/backups, and some are only included to one level deep, without sub-folders. The three suggested dirs should be backed up to all levels in Opus 13, but IconSets was only to one level in Opus 12 (and earlier 13 betas).
If in doubt, you can do a config backup and then rename the .ocb file to .zip to look inside it and check if your files are included.