This is expected behavior, as the specified user accounts are local to each machine. A user called "lan" present on workgroup computer LAN-001 is unrelated to a user of the same name on workgroup computer LAN-002. This explains why you must submit two images, one for each distinct user.
For the proposed example you need two user picture folders, "LAN-001" and "LAN-002", each with a lan.jpg inside.