When you work with graphics (including digital photographs) for web design, you work with two file types: raster and vector. When you create, edit, or manipulate those files, you use graphic- or photo-editing software to accomplish the job. Here's a list of some software that you may want to consider.

Software Program File Types
Adobe Photoshop Raster-based image-editing program used to create PSD layout files and to work with raster image file types and photography
Adobe Illustrator Vector-based image creation and editing used to edit vector image files, such as AI and EPS files
Corel PaintShop Pro Raster image and photography editing program
CorelDRAW Vector image creation and editing for AI, EPS, and CDR file types