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!
Imgur - Imgur is a free and simple image hosting service with image editing feature. Signup is optional.
IrfanView - IrfanView ... one of the most popular viewers worldwide.
Picorator - Picorator is a simple yet powerful image sharing application developed by online communities.
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
imgbox - Imgbox is a free image hosting service.
JPEGView - JPEGView is a small and fast viewer/editor for JPEG, BMP, PNG, GIF and TIFF images.