Getting Certificate Thumbnail using Invoke-command

Hi all -

I'm really new to powershell and working through things.  

So I need to get the thumbnail of a cert from a large list of servers and save it in variable, and then use it in the script later on.

I have tried this:

$thumb = Invoke-command -ComputerName $computername -Credential $cred -ScriptBlock { Get-ChildItem -Path Cert:\LocalMachine\My | select Thumbprint}

I have to be missing something, is there a way to just get the value of the thumbnail into the variable?

Thanks in advance for any help!