Introduction

First, it's important to understand that there are two types of image information: pixel-based and vector-based. Pixel based images are simply made up of a bunch of differently-colored square pixels, whereas vector-based images consist of mathematical formulas which describe the shapes and colors of the image. This means that vector-based images are much smaller, but they can't hold nearly as much color information as a pixel-based image. Which should you use? For the most part, most web graphics are pixel-based (with the exception of some Flash and Shockwave sites - they're largely made up of vector images). Some formats support both pixel-based and vector-based information in the same file, which will be discuss this below.

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GIFs are very small because they are limited to a 256-color palette, and are therefore the best way to go if you're worried about download time on a web page. Not the ideal format for saving photographs or anything that you want a lot of detailed color information in. This format uses a palette of up to 256 distinct colors from the 24-bit RGB color space. It also supports animations and allows a separate palette of 256 colors for each frame. The color limitation makes the GIF format unsuitable for reproducing color photographs and other images with continuous color, but it is well-suited for more simple images such as graphics or logos with solid areas of color.

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JPEGs are similar to GIFs, but they support millions of colors (which is what you need to make a photo look realistic). JPEGs use a higher compression rate than GIFs, which can be helpful, but if you compress a JPEG too much, the image will start to lose quality and detail. They tend to be slightly larger than GIFs, but if you use high-quality JPEGs the extra size is worth the better quality (see images below). JPEG typically achieves 10 to 1 compression with little perceivable loss in image quality. JPEG is the most common image format used by digital cameras and other photographic image capture devices, and is the most common format for storing and transmitting photographic images on the World Wide Web.

As you can see, JPEGs have their drawbacks if you're trying to save space. But as long as you stick to high-quality compression (most image-editing applications will give you an option box to select the quality when you save a JPEG), your images will look fine.

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PICTs are the Macintosh's native graphics format. It is one of the few image formats that will support both pixel- and/or vector-based images. PICT files are not compatible for Web use, so they're only useful for graphic design work. Unless you need both vectors and pixels in an image, don't use PICTs - JPEG's compression is much more efficient.

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PNG is the upcoming web-graphics format. It supports 16.7 million colors, uses lossless data compression, allows the exact original data to be reconstructed from the compressed data. PNGs still aren't widely used, however, any older version of Netscape and Internet Explorer does not support PNGs without a special plugin.

PNG was created to improve upon and to replace the GIF format, as an image-file format not requiring a patent license. PNG supports palette- based (palettes of 24-bit RGB colors), greyscale or RGB images. PNG was designed for transferring images on the Internet, not for professional graphics, and so does not support other color spaces(such as CMYK).

PNG is a better choice than JPEG for storing images that contain text, line art, or other images with sharp transitions. Where an image contains both sharp transitions and photographic parts, a choice must be made between the large but sharp PNG and a small JPEG with artifacts around sharp transitions. JPEG also does not support transparency.

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BMP is Window's native image format. Some applications (such as Photoshop) support up to 16 million color bitmaps. Bitmaps also support a type of compression that does not lose any image quality. They're useful to transfer high-quality images between applications that support bitmaps, but many applications do not.

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EPS is the main format for saving vector-only images that you can create in most drawing and page-layout applications (such as Illustrator and Freehand). It's important to note that the EPS format handles pixel information very inefficiently (and results in larger-than-necessary files), so only use EPS for vector-based images. It is generally considered to be one of the best graphic format for high resolution printing of illustrations due to it's vector-base nature that preserve the quality in a image.

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TIFFs is a flexible and adaptable file format due to it's support of numerious selection on different color spaces, compression type and pixel format. TIFF typically uses pixel-based format. Since TIFF compression does not lose any image quality at all, it is widely used in the publishing business to transport images. TIFF is also another prefered format for high resolution printing of images.