Great option to other image editors and image creating software. It has a simple and intuitive user interface, yet supports both layers and 3D objects. Functional across all major OS. It is free, as in open source. It imports .PDN files created with Paint.NET without issues.
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!
MyPaint - MyPaint is a fast, distraction-free, and easy painting tool for digital artists.
IrfanView - IrfanView ... one of the most popular viewers worldwide.
GIMP - GIMP is a multiplatform photo manipulation tool.
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
Microsoft Paint - Paint (formerly Paintbrush) is a simple, ultra-basic graphics painting program that is included...
JPEGView - JPEGView is a small and fast viewer/editor for JPEG, BMP, PNG, GIF and TIFF images.