The 'industry standard' photo-manipulation software (which is used by professional photographers, advertising agencies, magazines, etc) is Photoshop. The full version is fantastic but, with a £700 price tag, it ought to be!
However there's an open source (i.e. free!) programme which mimics the full version of Photoshop, so it's like getting 700 quids worth of software for nowt!
Here it is:
http://www.gimp.org/
Be warned though, using either Photoshop or Gimp requires a steep learning curve. (Colleges run full-time courses lasting several terms, or even several years, to train people how to get the best from Photoshop). If you're looking for something simpler, then Serif PhotoPlus Starter Edition might meet your needs. It's the free version of a full commercial product (costing over £70):
http://www.serif.com/...ware/?MC=FSSPHOTOPLUS
Chris