THM is a very small file which is often created by a camera in order to act as a thumbnail for an image and more often video. The file name has the extension THM. It is very easy to browse a series of images and videos at the time of playback without having to load the actual media file in the memory. Video has also the same name, which is saved as THM though there is a video extension which is named as AVI.
You can represent a THM file as first frame of video. It is the initial image you see when previewing a movie in a camera. In the case of videos, THM is often used to store data much in the similar way as is done with EXIF data for still images. It happens because a THM file is actually a small JPEG with a different extension.
The file extension JPG is primarily associated with a 'JPEG/JIFF Image', which is nothing but a lossy bitmap image format that was used by all digital cameras to display photographs on the Web. If you wish to edit a JPEG file, first you have to save it as a BMP or other lossless bitmap format. After that edit it and then use "Save As" option to resave it as a JPEG file. If you go to edit and reedit a JPEG image, you will suffer for a compression loss on each read and save segments.
If you change the THM extension to JPG, the file will be converted to JPEG.For example, if you change a file named as MVI_8233.thm, it will be converted to a file named MVI_8233.jpg. Most cameras will not be able to play back videos if a THM file is missing or damaged.
Source by Paul N. Murrey