An STN file is a scaled image saved in the STiNG format and created by Genuine Fractals, an image editing plugin for Photoshop. It is a resolution-independent copy of a raster image, allowing it to be scaled without loss of image quality.
Genuine Fractals was originally developed in 1996, acquired by LizardTech in 2001, then purchased by onOne (now ON1) Software in 2005. It was incorporated into their Perfect Resize software, which was later renamed to Resize.
Unfortunately, STN files are no longer supported by ON1 products. The only way to open STN files is by using Adobe Photoshop with Perfect Resize 7.5 (no longer available) or earlier installed. In order to view your STN files you need to convert it to another format such as JPG-2000, which is more of an industry standard that still supports wavelet compression.