You can recover images, files and attachments deleted in this manner for 30 days (then they get deleted automatically) and whilst they remain temporarily deleted they still use space on your site.
If you permanently delete these files they become completely unrecoverable and they space they use becomes available.
Google Sites - Basics - Delete Images or Attachments - Recover Spaceposted 15 Jan 2012, 13:24 by Stephen Hind [ updated 5 Jun 2014, 08:49 by Unknown user ]
These instructions tell you how to delete any images, files or attachments you upload to your site, releasing the space they use. At the moment Google Sites does not provide a means to delete images uploaded to the sidebar, so I suggest you always upload your images to a page (then add them to the sidebar by web address) so you can delete them if you need to.