About Images
Many different types of graphic file formats exist, but three graphic file
formats are generally used in web pages -- GIF, JPEG, and PNG.
Currently, GIF and JPEG file formats are the best supported and can be viewed by most browsers.
PNG files are best suited for almost any type of web graphic due to their flexibility and small file size; however, the display of PNG images is only partially supported in Microsoft Internet Explorer (4.0 and later browsers) and Netscape Navigator (4.04 and later browsers).
So unless you are designing for a specific target audience using a browser that supports the PNG format, use GIFs or JPEGs for broader accessibility. GIF (Graphic Interchange Format) files use a maximum of 256 colors, and are best for displaying noncontinuous-tone images or those with large areas of flat colors, such as navigation bars, buttons, icons, logos, or other images with uniform colors and tones. JPEG (Joint Photographic Experts Group) file format is the superior format for photographic or continuous-tone images, because JPEG files can contain millions of colors.
As the quality of a JPEG file increases, so does the file size and the file download time. You can often strike a good balance between the quality of the image and the file size by compressing a JPEG file. PNG (Portable Network Group) file format is a patent-free replacement for GIFs that includes support for indexed-color, gray scale, and true-color images, and alpha channel support for transparency. PNG is the native file format of Macromedia Fireworks.
PNG files retain all the original layer, vector, color, and effects information (such as drop shadows), and all elements are fully editable at all times. Files must have the .png file extension to be recognized as PNG files by Dreamweaver.
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GIF, or Graphics Interchange Format, is a popular web graphic format. GIFs contain a maximum of 256 colors. GIFs can also contain a transparent area and multiple frames for animation. Images with areas of solid color compress best when exported as GIFs. A GIF is usually ideal for cartoons, logos, graphics with transparent areas, and animations.
JPEG was developed by the Joint Photographic Experts Group specifically for photographic or high-color images. JPEG supports millions of colors (24-bit). The JPEG format is best for scanned photographs, images using textures, images with gradient color transitions, and any images that require more than 256 colors.
PNG, or Portable Network Graphic, is a versatile web graphic format. However, not all web browsers can view PNG graphics. A PNG can support up to 32-bit color, can contain transparency or an alpha channel, and can be progressive. PNG is the native file format for Fireworks. However, Fireworks PNG files contain additional application-specific information that is not stored in an exported PNG file or in files created in other applications.
WBMP, or Wireless Bitmap, is a graphic format created for mobile computing devices such as cell phones and PDAs. This format is used on Wireless Application Protocol (WAP) pages. WBMP is a 1-bit format, so only two colors are visible: black and white.
TIFF, or Tagged Image File Format, is a graphic format used for storing bitmap images. TIFFs are most commonly used in print publishing. Many multimedia applications also accept imported TIFF graphics.
BMP, the Microsoft Windows graphic file format, is a common file format used to display bitmap images. BMPs are used primarily on the Windows operating system. Many applications can import BMP images.
PICT, developed by Apple Computer, is a graphic file format commonly used on Macintosh operating systems. Most Mac applications are capable of importing PICT images.