IrfanView is certainly one great freeware tool and I personally recommend it to anyone. IrfanView though is hardly an editor, and outside of cropping, resizing, converting to a slew of professional graphic file formats and applying image filters it does not resally give you the tools to "edit" a digital image.
For some great free image editing tools, these are my personal recommendations:
part of 602PC Suite a fully working Microsoft Office suite clone that includes a full image editor compleytely free - no spyware
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Serif Photoplus 5.5
This is a little Photoshop, including layers, transparency, clone tool, image optimization tools and a lot more. The tools is not difficult to use and the interface is rather simple. There a few bugs here and there but the tool is more than usable. Serif commercializes version 9 of this tool today and distributes the older version for free to let its brand get known. (Nice marketing no?)
donwload it here: http://www.freeserifsoftware.com/serif/ph/ph5/index.asp
Oriens Enhances is a third possible alternative.
I do not recommend GIMP if you are a casual user.
Here you can find some more information:
"Best low-cost and free image editors"
"Feature-rich Free Digital Image Editor"