@bes The directory stores all photos or other uploads to the GNU Social instance. If you only want to have the thumbnails from OStatus profile posts on the server you can activate "Oembed". This plugin prevents GNU Social from downloading the images on your server and is linking to the source instead.
If you delete the files in the directory all photos in the notices and all thumbnails of them, links, etc. are gone. So you may can delete older photos

!gnusocial