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bitmapped (or rasterised) graphics
When producing new versions of a graphic from the original,
always use the original to make each new version. This
will retain as much of the quality as possible. And
always ensure that the image mode is on rgb color.
This is because each time a graphics package shrinks
an image, it reduces the number of colours, therefore
reducing the quality. So if you use the original every
time, you will always get the most colours possible.
This also means that unless the image is a vector image,
you can't create a good quality larger version as there
won't be enough colours to render the image at the same quality.
Once a graphics package, such as Adobe
Photoshop or Macromedia
Fireworks, has optimized the graphic, you cannot
get back to the original quality. Gifs are indexed color
and jpgs may have have their quality reduced to shrink the
There will always be the sacrifice between quality and
file size - smaller file size = quicker download times.
re-sizing images in a web page
Never use the width and height attributes of
the <img> tag in the html of a web page to resize
images. If you use this method to reduce the image size, you
will increase the download time of the page as a whole.
This is because the browser has to "re-draw"
the image at an incorrect size which takes longer.
In addition, never use this technique to increase the
size of an image - it severely reduces image quality
as well as taking longer to download. Use you favorite
to re-size your images properly and include the correct
width and height dimensions for a speedier download.
This means your site will look better, and your visitors
will be happy with a quicker download time.