How to change a logo across all sites/sub-sites and pages?

Add-PSSnapin Microsoft.SharePoint.PowerShell -erroraction SilentlyContinue 

$SiteURL = "http://Your-Site-Collection-URL" 
#Get the site collection
$Site = Get-SPSite $SiteURL #Iterate through each site in the site collection
 foreach($web in $Site.AllWebs)
 {
 #sharepoint change logo programmatically 
$web.SiteLogoUrl = "/relative-path-logo-file" 
$web.Update() 
}