This is a free open-source image editor that I have used for a long time. It works on all the major platforms (Windows, Linux and Mac) and is very light weight consuming very little resources. XnView will not replace Painter or Photoshop but can be used for quick editing, batch editing and converting along with many other features.
XnView can be download from http://www.xnview.com/en/
In this short tutorial I cover how I use XnView, which is mainly to browse and organize my images and copy / cut thumbnails from full page scans allowing me to keep what I like.