Image editing software is essential to web design. Aside from the pure HTML code, you will almost always need graphics to go with your web page.
Here, the most commonly used imaging tools are listed:
Bitmap editors
Photoshop
Photoshop is the industry standard, but comes with a high price tag.
GIMP
Many consider the GNU Image Manipulation Program, or GIMP for short, to be the open-source Photoshop. Although it lacks some features of the program it asprires to replace, the GIMP is being constantly improved, and many use it professionally for web graphics.
Paint.net
Another open-source program for editing raster graphics.
Vector editing
Inkspace
Like GIMP, Inkspace is a popular open source replacement for proprietary and expensive imaging software.