I use Adobe Photoshop. It costs iro 500/600 squid, but you can get a cut-down version called Photoshop Elements for a lot less.
It's as user friendly as you want to make it really. By that I mean it's a gargantuan programme - as someone who's used it professionally for 7 odd years, I can say I probably only ever use 40% of its features - but if you only want to scratch the surface it's not too difficult. Plus there are bucketloads of great manuals out there. Photoshop "wow" books are good, as too is the "Photoshop Bible".
Download a demo (
www.adobe.com), get a manual out of the library and have a bash.