PhotoQt is a fast and simple, good looking, yet powerfull and highly configureable image viewer.
It provides an uncluttered interface and can be extensively tweaked to fit ones needs perfectly. It is possible to set key or mouse shortcuts for almost any one of the features, plus for any external script/command one might want to run.
The many features provided include: thumbnails, slideshow, chromecast, zooming, rotating, flipping, displaying metadata (selection of Exif and IPTC), set-as-wallpaper, simple file management, scaling, face tags (XMP metadata), integration with many image libraries for wide support of image formats, support of touchscreens, and a lot more...