Why we need to optimise our Images?

For Speed and Quality!

If all we do to resize our image is simply drag to a new size we will not be creating good web pages. Professional Designers will often create pages in this way.
But: They will use specially prepared Black & White low-resolution image.
Having arranged the page layout and just before publication the “Black & White low-resolution positional image” is then exchanged for the final image, sized and optimised.

What size should an image be?

There is not an exact answer to this question.

The variables to consider are:

• The number of pixels in relation to the screen size.
• The amount of file compression.
• The speed and method of transfer.

Why can't we mathematically calculate the size required?

• Variations in display size.
• PC uses 96 dpi and the Mac uses 72
• The number of colours that can be displayed.
• The size of the display screen.
• 640 x 480
• 800x600
• 848x480
• 1024x768
• 1152x864
• 1280x1024
• 1600x1200
• The Browser being used to view the page.
• Internet Explore / Netscape / Opera / NeoPlanet / ….
• The Version or Build V4 / V5 / V5.5 / V6  
• User Options, Tiled or Full Screen.
• Open Tool Bars.
• Start Menu and Task Bar.
• Microsoft Tool Bar.
• Auxiliary Tool Bars for Video or Sound.
• Browser Tool Bars and  Buttons.