Zept:If you have plenty of money, plenty of RAM and plenty of time to learn how to use it, Adobe Photoshop is unequalled -- but it's one serious piece of software. If you can stomach a 42Mb download, you can get a 'tryout' version from http://www.adobe.com/
Paint Shop Pro is a popular and cheaper alternative. Evaluation version (a mere 32Mb) at http://www.jasc.com/
Second.
Also, if you need sometime that can give you a quick fix on bulk, I'd like to recommand another free s/w, Picasa2.