Irfanview is one of the best graphic file viewer and converter. I would try it first – it’s free. http://www.irfanview.com/ THM files are usually thumbnail previews of other graphic files and they are usually created within the graphics program you are using or by a digital camera. THM files are really just small jpg files. You can rename them interchangeably and use any image editing program. When you try to open a THM file you may get a warning or Windows may not recognize the file type. Irfanview does open THM files, but with a warning.