Slow-scan television is an image file format whose origin is unclear, though it's undoubtedly related to the Slow-scan television (SSTV) image transmission method used in amateur radio.
Its name is punctuated and capitalized in various ways, sometimes with a capital V: "Slow Scan TeleVision". It might also be called HRZ.
The image format is a simple RGB raw bitmap format. The image dimensions are 256×240 pixels. A file is 184320 bytes in size.