Steegle.One Intranet for Google Workspace
For new Google Sites there is no quota for image size and files are stored in Google Drive (see that quota instead). Google has imposed a 1,000 page limit and 200,000 character limit per page and an overall 10 million character limit per new Google Site - see Files you can store in Google Drive.
Sign in to your Google Site
Use the More button (looks like a gear/cog) and then Manage Site(or keyboard shortcut g then m) and go to Attachments.
You will see a list of all the images, files and attachments available on your Google Site: find the file or files you wish to delete and check the checkbox on the left of the file(s).
Use the Delete button at the top of the page and the files will disappear from the list.
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.
or
Sign in to your Google Site
Use the More button (formerly More Actions) and then Manage Site and go to Deleted items
You will see a list of all the images, files and attachments available for recovery or permanent deletion: find the file or files you wish to recover or permanently delete and check the checkbox on the left of the file(s).
Use the Recover button to restore the deleted file back to its original location or the Delete permanently button to delete the file forever and recover the space for your site.
If you permanently delete these files they become completely unrecoverable and they space they use becomes available.