As I’ve noticed, NVIDIA uses dds format a lot, when openning it with notepad it seems to be some kind of DXTC5 format. The question is how can I open this file with graphic editor (which ones suport dds) or how to load it in C.
DevIL has dds support. Source is available, so you should find the loading code you look for. http://www.imagelib.org
Regarding editors, I think there are some dds loading plugins around for the most popular ones (eg Photoshop). I don’t know if there is any editor with built-in dds support though.