Thumbnails for Radio Programmes

[Budgie]

I have just transferred about three month's worth of downloads to my NAS 
media server where they are served by minimserver.  Sadly it looks as 
though I have messed up, as none of the files has a thumbnail showing 
whereas all almost all the previously saved radio programmes do have a 
thumbnail which shows when I view library from renderer control device.

Questions:-
Is there any easy way to check the .m4a file to see if a thumbnail is 
included?
What should my prefs be to ensure download includes thumbnail and that 
it is included within the file?

I confess I had this problem before but cannot access the old email 
messages as I am still trying to get legacy SCSI card to work with new 
mobo.  If anyone has time I would be grateful for help.
Regards,
Budgie