There are four most common image files which are used in design today, these being – JPEG, GIF, TIFF and PNG. These image files have their own individual ways of storing colour modes. A JPEG image using RGB mode will store 24 bits per pixel or 8 bits per pixel in Grayscale mode.
A GIF image file provides indexed colour at 1 or 8 bits per pixel. A TIFF image using RGB mode will store 24 or 48 bits per pixel or 8 or 16 bits in grayscale mode and at indexed colour holds 1 or 8 bits per pixel. A PNG image file using RGB mode will store 24 or 48 bits, grayscale 8 or 16 bits and indexed colour 1 or 8 bits.
Photographic images usually have continuous tones within the image, this means that pixels that are positioned close together usually have similar colours, and for example, a photo of green grass will contain numerous shades of green. A JPEG photographic image is usually 24-bit RGB colour, or 8 bit grayscale, and a typical colour photograph may contain around 100,000 colours, out of the possible set of 16 million colours in 24-bit RGB colour.
Website pages require JPG, GIF or PNG image types, because that is all that online browsers can show. JPG is the best choice on the web for photo images as it is the smallest sized file and website pages tend to use the GIF format for any graphic images e.g. logos or line art.