Keeping this in mind, we can move onto the next section - file size and compression. Although we usually say an image has 16 or 256 or 16.7 million colours, most people who work with graphics talk in 'bits per pixel'. A bit is the most basic unit of computer measurement. Eight bits is equal to one byte. If someone says they have an '8-bit image', they mean it has 256 colours.
Compression means their file size can be dramatically reduced.
Compression Compression is a way to collapse the pixels in a file down so that the size of the image takes up less disk space. Basically, the way compression works is really simple. Go back to the example before. Imagine telling someone how to draw a picture by saying 'red, red, green ' and so on. Well, imagine that you came across 18 red pixels in a row.