Knowledge of editing photo, photo editor software,
photo retouching, photo design & Photoshop
Price of photo editing software

There are several very good programs out there.  The next question is how much do you want to do with it. 

Cropping and fixing red eye can be done with almost any freeware. 

Adobe Lightroom is basically a digital darkroom which allows you process photos for exposure, white balance, minor touchups etc, but not real heavy photo manipulation. Photoshop Elements is a scaled down version of the main program(much less expensive that full PS).  And then Photoshop.  If you're looking for professional software, Photoshop is probably the way to go.  If you're just looking for basic photo program, check out ACDSee or 5DFly photo design.  It's a great program for a great price.

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