Image editing software allows you to make changes to graphics, as well as create your own ones. Any basic image editing software will allow you to make simple enhancements such as adjusting the brightness and contrast to make your photo more vivid. More advanced features in some graphics programs will allow you to apply a variety of special effects.
Adobe Photoshop
Adobe Photoshop is regarded by most as the industry standard image editing software, and offers many advanced graphics enhancement features. Although initially meant mainly for print, Photoshop is increasingly being used to edit images for the web. Photoshop integrates with the ImageReady application to ease the creation of web graphics. Photoshop is available for Microsoft Windows and Mac OS X.
One interesting feature is the "Save for web" feature in Photoshop. It optimizes images, meaning that image size is kept at the minimum while preserving image quality.
Photoshop tutorials can be found at:
Good Tutorials
Absolute Cross
Adobe Photoshop Elements
You may download a 30-day trial of Adobe Photoshop Elements, essentially a toned down version of Adobe Photoshop. Photoshop Elements caters to those who do not require the advanced features of Adobe Photoshop. It is nonetheless still useful and available at a relatively affordable price. Photoshop Elements is available for Microsoft Windows and Mac OS.
Adobe Fireworks
Adobe Fireworks is designed specifically for web-based graphics. Fireworks offers vector editing capabilities. It is available for Microsoft Windows and Mac OS.
GIMP
Download GIMP.
General Image Manipulation Program (GIMP) is an open source image editing programme which allows you to do image editing. GIMP is available for Microsoft Windows, Mac OS as well as Linux.