Could you please post both these URIs? Perhaps they contain some clues on where the WMP Network Sharing Service tries to get the album art from.
Here is the good one:
http://192.168.1.33:10243/WMPNSSv4/813663920/e0VENkY5NjRBLTFBQjAtNDU2NS1BNDRDLTgxMDNFQUIzOTZGNX0uMC40.jpg?albumArt=true,formatID=13
And the bad one:
http://192.168.1.33:10243/WMPNSSv4/813663920/ezhGRDM4NTRDLTcyQjUtNEVCMS04MTYyLUY4Q0UwNkZCRUUzM30uMC40.jpg?albumArt=true,formatID=13
And here is a trace of an HTTP GET on the bad url...
GET /WMPNSSv4/813663920/ezhGRDM4NTRDLTcyQjUtNEVCMS04MTYyLUY4Q0UwNkZCRUUzM30uMC40.jpg?albumArt=true,formatID=13 HTTP/1.1
Accept: text/html, application/xhtml+xml, */*
Accept-Language: en-GB
User-Agent: Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)
Accept-Encoding: gzip, deflate
Host: 192.168.1.33:10243
Connection: Keep-Alive
HTTP/1.1 200 OK
Content-Length: 9075
Content-Type: image/jpeg
Server: Microsoft-HTTPAPI/2.0
Accept-Ranges: bytes
TransferMode.DLNA.ORG: Interactive
Date: Thu, 17 May 2012 12:31:09 GMT
...returns the WMP default "music note" image...