An image editor is the tool you need for working with photographs and bitmap graphics. In addition to strong photo-editing features your image editor should easily import and export common graphics formats including TIFF, GIF, JPG, and EPS. For professional design work, Photoshop and Photo-Paint are the industry standards.
The most widely-used professional image editing program has a hefty price tag but it's packed with features. Photoshop is almost a prerequisite for most professional design employment.
Popular among both experienced and novice Windows designers it is becoming increasingly popular for professionals as a less-expensive but feature-rich alternative to Photoshop, especially when you consider that it comes as part of the powerful CorelDRAW Graphics Suite.
Combining natural media painting tools with more traditional image editing features, Painter is truly a digital artists studio.
Sometimes called the poor man's Photoshop it's a powerful program in its own right -- for Windows only. Available for about $100US, it has many of the same or similar features as the high-end programs.