GIMP - GIMP is a multiplatform photo manipulation tool.
digiKam - Professional Photo Management with the Power of Open Source
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
XnView MP - XnView is a free software that allows you to view, resize and edit your images. It supports more than 500 different formats!
Krita - Krita is a professional FREE and open source painting program. It is made by artists that want to seaffordable art tools for everyone. Concept art. texture and matte painters, illustrations and comics.
FastStone Image Viewer - FastStone Image Viewer is a fast, stable, user-friendly image browser, converter and editor.