File Extension THM
File extension THM is most commonly used by many different applications to store THuMbnails. Thumbnails are small images that are reduced size versions of larger images or videos. Thumbnails are usually created for browsing multimedia libraries. Thumbnails take less processing resources to display than full-sized pictures and much less bandwidth when they are transferred over the Internet. Some programs group multiple thumbnails together in single file. This is done to avoid unneeded strain on computer file system. Thumbnails may also be used for vector graphic formats that would be extremenly hard to render in real-time.
Although thumbnail data is usually encoded using some standard algorithm like JPEG, TIFF or just plain bitmap each application would use it's own custom container format. Because of this thumbnail files usually can only be opened by application that created them. Also most thumbnail files can not be opened directly. Instead they are indirectly used by software when you browse the image catalog.
File extension THM may also be used by OpenOffice software package to store it's configuration information. OpenOffice is a freeware alternative to Microsoft Office that provides similar functionality.