Adobe Photoshop is number one program when we talk about professional image editing. Unfortunately it is expensive enough. Though there are free alternatives even to Photoshop. Lot of people choose free open source software GIMP instead. GIMP is pretty equal with Adobe Photoshop in functions.
For GIMP users there is good book written by Carey Bunks and published by New Riders – “Grokking The GIMP“. The online version of this book is free and is distributed under the Open Publication License. This book will guide you to learn about GIMP. The book will teach you from basic tools of GIMP : layers, selections, masks, and colorspaces and then presenting in depth of touchup, enhancement, compositing, rendering, and image creation for the web.