Problem: Cannot see .jpg files on a Windows 2003 Enterprise server shared (SMB) folder.
I spent a great deal of time working on this today. I wasn't able to find any posts on xbox-scene regarding this issue, so now that I have resolved the problem, hopefully I can share some valuable information. I finally found the solution (posted in a different context), on samba.org.
Solution: Windows Server 2003 is more secure than any of the previous Windows OS's. Windows Server 2003 requires signature of SMB packets by default. In order to disable the digitally signed communications you need to make the following registry hack on your Windows Server 2003:
Key: HKLMSYSTEMCCSServicelanmanserverparameters
Value: RequireSecuritySignature
Content: 0 to disable, 1 to enable
Reboot the server.
Good luck!