XnView MP stands for Multi Platform = it is available for Windows, Linux and MacOS. It is a continuation of the older XnView Classic. In my opinion it is a great alternative to Windows photos app or any Linux image viewer that comes preinstalled. I even think it is an alternative to IrfanView. It is really fast and simple to use, supports more than 500 image formats, has built in folder browser, image converter, editor... AND MORE!
Photoscape - high quality photo editing software, that enables you to fix and enhance photos.
IrfanView - IrfanView ... one of the most popular viewers worldwide.
ACDSee - ACDSee is an image viewer that uses very little system resources, and it has a small, sleek size that won't take up much space on your system.
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
JPEGView - JPEGView is a small and fast viewer/editor for JPEG, BMP, PNG, GIF and TIFF images.
Zoner Photo Studio X - Zoner Photo Studio X is a full RAW editor and image organizer software. It features a bit-map editor with layer and mask support, a video editor and a full module dedicated to culling, cataloging and complex metadata management.