I think that feature could use some UI improvements to help users understand what is going on.
As a user, you should be able to fully delete any media that you have uploaded. Go to your profile, then the uploads tab. For your account, it will be on this link: https://lemmy.ca/u/ColemanLaing?page=1&sort=New&view=Uploads (other users can't see this page)
However, users can't permanently delete posts without help from site admins. I imagine this has to do with preventing harmful content, since otherwise users could post spam and then delete it without the admins from being able to see what they did.
Admins can "purge" posts from the site, in a way that it gets deleted from the database. This is what it looks like on my interface:
