Graphic images do not normally have a continuous tone unless a gradient has been used within the graphic. Graphics are drawings are not like photos plus they usually use few colours, less than 16 colours in the whole image. In a colour graphic cartoon, a particular area of colour will use one shade, where as in a photograph there may be numerous shades of one colour.
A map is produced using graphics and only uses 4 - 5 map colours plus 1 - 2 colours of text and then blue water and white paper, so these types of graphics use less than 16 colours, Graphics like this are ideal for Indexed Colour.
The TIFF file format is the best image file to use when best quality is required, and this is why the TIFF is common in professional and commercial printing environments. High Quality large JPG images are also good too, but they can be ruined if they are made too small. The 2D digital image is split into two parts, images know as ‘bitmapped’ are usually used in image making programmes such as Photoshop or painting packages, bitmap images are usually made up of rectangle picture elements known as pixels and each pixel is a colour, if the image is enlarged you can see these pixels and the image appears jagged, this can be improved by increasing the number of pixels per inch, known as a higher resolution image.
The other part is known as a ‘vector’ image and these are used in drawing and illustration programmes like Adobe Illustrator, a vector image is made up using lines and shapes, if the vector image is enlarged the quality will not degrade and the smoothness of the final image is only determined by the output device used to print the image.