Adobe Photoshop, a graphics editing program, provides a large library of effects, filters and layers. It is the industry standard image manipulation tool and one of the most versatile and powerful programs available to the designer today. It is essential for a professional finish. Photo manipulation is the application of image editing techniques to photographs in order to create an illusion or deception (in contrast to mere enhancement or correction), through analog or digital means.
Here are a a few tutorials that cover specific topics that you may find of interest: