thumbs.db definition, relationships, removal:

This is a part of a malicious parasite that violates user privacy and damages the compromised system. The thumbs.db file can be used for different purposes depending on the related parasite's type, functionality and carried payload. Although thumbs.db may not take any malicious actions by itself, it ensures that a parasite is working properly and therefore needs to be removed. DO NOT open, execute or edit thumbs.db!
The thumbs.db file is installed and used by Fade. It is also related to other parasites such as Mosucker, Neoturk and NeuroticKat. These threats can use thumbs.db for different purposes, as the file's functionality depends on a particular parasite it belongs to.
Sometimes a presence of thumbs.db does not indicate that your computer is infected. It can be an essential system file or a file installed by harmless legitimate software and therefore may not pose any threat to your privacy and the system. Although such cases are quite rare, you should thoroughly examine thumbs.db before completely deleting or leaving it in the system.