A bitmap image is simply an image that is constructed from an array of pixels. In a bitmap, each binary number representing a pixel’s colour is mapped directly to a set of coordinates in an x, y graph.

If you right click on the title bar of an image in Photoshop, and then select Image Size from the menu that appears, you will display the image size dialog box. In the top part of this box you can see width and height values under the heading Pixel Dimensions.

 In this example, the width of the image is 424 pixels, and the height is 640 pixels. This means that there are 424 x 640 =271360 pixels or individual tiny blocks of colour in the image. Each of these pixels is assigned to a position in the 424 by 640 rectangle that makes up the picture. One pixel will be mapped to location (125 across, 300 down), and another will be mapped to (410 across, 635 down). In this way, the colour value for each pixel is aligned correctly to produce the image that you see on your screen. 

GIF, JPEG, TIFF, and PNG files all use this strategy, and are therefore considered to be bitmap image types.