IrfanView
IrfanView is a free image viewer and editor that we’ve used (and really enjoyed) since the Windows 95 days. It is quick, light and easy to use. This YouTube clip is a good basic tutorial on how to get started with IrfanView:
Samuel Dean has written a couple of great posts about IrfanView on the Web Worker Daily blog called IrfanView: Great Graphics On-the-Fly, and Free, and Tutorials and Plug-Ins for IrfanView–a Graphics Gem.
Ben Carigtan has written an outstanding tutorial called Photo sketch tutorial and swapping colors using IrfanView for the Online Tech Tips blog.
A Google search for IrfanView Tutorial will give you many more guides and tutorials to choose from.