digiKam - Professional Photo Management with the Power of Open Source
ShareX - ShareX is a free and open source program that lets you capture or record any area of your screen...
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
Greenshot - Greenshot is a free and open source screenshot tool that allows annotation and highlighting using the built-in image editor.
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.
LightShot - The fastest way to take a customizable screenshot.