Different kinds of images
For the vectorial graphics, an image is a set of graphic basic elements that describe dots, lines, curves and polygons to which colors and shades (halftones) can be applied. Therefore also a vectorial image may contain some shades to be printed
Example of shades in vectorial graphics
Raster images or Bitmap
The word raster comes from the analogical television, meaning the horizontal lines (scan lines) of the TV sets and monitors.
For the raster graphics the images are described as a grid of colored pixels ( for instance using 256 colors or more ). For the computer graphics raster is the orthogonal dots grid that constitutes a raster image.
For raster graphics the image is seen as a chessboard and to every element, called pixel, is given a specific color.
From a technical point of view, the raster or photographic image is reproduced in screen printing and in T-shirt printing through four-color process or through the print of a polychromy in screen printing