Remove all media items that are not referenced.

RemoveMediaItems.ps1

<#
    .SYNOPSIS
        Removes all media items that are not linked to other items.
#>

# HasReference determines if the specified item is referenced by any other item.
function HasReference {
    param(
        $Item
    )
    
    $linkDb = [Sitecore.Globals]::LinkDatabase
    $linkDb.GetReferrerCount($Item) -gt 0
}

<# 
    Get-MediaItemWithNoReference gets all the items in the media library
    and checks to see if they have references. Each item that does not
    have a reference is passed down the PowerShell pipeline.
#>
function Get-MediaItemWithNoReference {
    $items = Get-ChildItem -Path "master:\sitecore\media library" -Recurse | 
        Where-Object { $_.TemplateID -ne [Sitecore.TemplateIDs]::MediaFolder } |
        Where-Object { $_.TemplateID -ne [Sitecore.TemplateIDs]::Folder } 
    
    foreach($item in $items) {
        if(!(HasReference($item))) {
            $item
        }
    }
}

Get-MediaItemWithNoReference | Remove-Item -Recurse