An image viewer, converter and even a very limited image editor. Using Castle Game Engine. It has many nice features, including support for some "exotic" file formats: DDS and KTX (advanced formats for textures) and RGBE (simple HDR format).
Complete documentation on https://castle-engine.io/castle-view-image.php .
Using Castle Game Engine, see https://castle-engine.io/ .
GNU GPL >= 2.