The three most common formats we see used on web and portal sites are:

JPEG/JPG Joint Photographic Experts Group - Most Popular
GIF Graphics Interchange Format - Great for line drawings
PNG Portable Network Graphics
 
JPEG / JPG
 
JPEG/JPG is the most popular and ideally suited for compressing photographic images. It has been designed so once the image has been compressed you can not see any difference in quality with the human eye. However if you do zoom in on an image that has been compressed and compare it to the original you will be able notice a difference, so the more you compress the image to try and save file size the lower the quality of the image will be. When you save your image as a JPG you will be given the option of what quality you want your final image to be, the higher the quality the larger the file size will be. You need to decide what point you can start to see a visible difference in quality, as a rough guide this is normally around 60%.

JPEG compressed file format supports 24 bit color (millions of colors). This is the best format for photographs to be shown on the web or as email attachments. This is because the color informational bits in the computer file are compressed (reduced) and download times are minimized.

JPEG compression ratio and resolution are not relational. As an example, you can have a high resolution JPEG image with either low or high compression. Higher JPEG compression equals lower image quality since the color information in individual pixels is compressed into 'blocks' of pixels using math algorithms that methodically blend all the pixel colors in each block. Increasing the compression produces smaller computer file sizes in kilobytes or megabytes. Lower compression produces better quality but bigger computer file sizes. The amount of compression you select at the time you create the JPEG image file determines the actual file size in kilobytes and the overall visual quality of the file. Overly compressed JPEG files may look a little soft or blurry. A quality option is usually available when you save pictures in your digital camera as JPEG. You will also select a 'quality' option when
you save files as JPEG in your image editing program.

JPEG supports embedded file information, including calibrated color space, output device profile information and digital camera exposure data known as EXIF data. EXIF stands for Exchangeable Image File Format. This data includes technical information about each photograph including shutter speed and aperture used, whether or not flash was used, the date the photo was taken. Scanner's may embed EXIF information too. Informational fields vary with each camera, photo-telephone, or scanner manufacturer.
 
GIF:

GIF compresses the image by reducing the number of colors, so the less colors you have the smaller the file size will, but having less colors also means that the quality of the image will be lower. GIF images are best suited for simple line drawings, logos and text. Another advantage of using the GIF format is being able to set certain colors within your image to be transparent. So that when your image is placed on a colored background, that background color will show through on the transparent part of your image. GIF is an uncompressed file format that supports only 256 distinct colors. It is best used with web clip art and logo type images. GIF is not suitable for photographs because of its limited color support.

PNG:

PNG which is pronounced PING was designed as a replacement for GIF and is the newest image
file format on the Web, and as a result is not completely supported by all browsers yet. It is still best to use either JPEG or GIF images instead. The biggest advantage of using PNG is that you can use this format to save photographic images and also be able to select certain colors as transparent. However the disadvantage is that when the image is compressed the file size is considerable larger than both a JPG or GIF. So at present the best time to use a PNG would be when you need to insert an image with a transparent background into another document, like a PowerPoint of Word document.

Additional Digital Image Formats

TIFF:

TIFF is an uncompressed file format with 24 or 48 bit color support. Uncompressed means that all of the color information from your scanner or digital camera for each individual pixel is preserved when you save as TIFF. TIFF is the best format for saving digital images that you will want to print. Tiff supports embedded file information, including exact color space, output profile information and EXIF data. There is a lossless compression for TIFF called LZW. LZW is much like 'zipping' the image file because there is no quality loss. An LZW TIFF decompresses (opens) with all of the original pixel information unaltered.

BMP:

BMP is a Windows (only) operating system uncompressed file format that supports 24 bit color. BMP does not support embedded information like EXIF, calibrated color space and output profiles. Avoid using BMP for photographs because it produces approximately the same file sizes as TIFF without any of the advantages of TIFF.

RAW:

Camera RAW is a lossless compressed file format that is proprietary for each digital camera manufacturer and model. A camera RAW file contains the 'raw' data from the camera's imaging sensor. Some image editing programs have their own version of RAW too. However, camera RAW is the most common type of RAW file. The advantage of camera RAW is that it contains the full range of color information from the sensor. This means the RAW file contains 12 to 14 bits of color information for each pixel. If you shoot JPEG, you only get 8 bits of color
for each pixel. These extra color bits make shooting camera RAW much like shooting negative film. You have a little more latitude in setting your exposure and a slightly wider dynamic range. Also, you also do not need to worry about setting the correct white balance. White balance, color balance, sharpness and exposure (+ or - one to two stops) are adjustable during the RAW conversion to TIFF or JPEG. The down side of RAW is that you DO have to convert the proprietary raw file to a TIFF or JPEG before your image editing program can open it. All camera manufacturers that make cameras that output RAW provide a RAW converter with their supplied software bundle. You can also buy very full featured aftermarket RAW converters. Camera RAW files converted to 16 bit TIFF produce the absolute best quality image available from any digital camera. Camera RAW supports imbedded EXIF data.
 
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