@moan
When you delete an image from a folder in the file manager, it winds up in the trash. Open the 'trash can' tab and you can permanently 'empty' the trash, however this simply makes the files un-restorable. To remove the files from the disc, you need to select the 'ADMIN' menu and choose 'Remove delete files' from the sidebar menu. This *moves* the files from 'PHPWCMS_Filestorage' into the 'can be deleted' folder.
I would assume that this would also remove the files from any WYSIWYG editor plugin list of available files, assuming the plugin was configured to look in PHPWCMS_Filestorage.
Your client can perform this housekeeping process in the backend, provided they have administrator status. It is up to you to periodically delete the files from the 'can_be_deleted' folder, either via FTP or a cron job.
But wait... there's more
When you USE an image in an article via a content-part, a copy of the image is created by PHPWCMS at the desired dimensions and placed in the 'content/images' folder. This is the image that is linked to the article. That's why even if you delete images from the file manager, they will still appear in articles.
Fortunately, when you open a web page containing an image, the re-sized version in the content/images folder is re-created dynamically from the 'master' copy in the file manager. This means you can safely empty the content/images folder periodically to 'purge' un-used re-sized images. The ones that are still in use will be regenerated 'on the fly'.
Once again, you will need to delete the folder contents via FTP or a cron job.
HTH.