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Alternatively you could place the image in a table set to 100% then fill the background the with the colour of part of you image so it blends from image to background.
Another trick I use on one site is to place the image in the background of a table cell,then repeat the image as a mirror on the next cell and set it to 100%. This will only work on 'banner' type images but can look very tasty.
It's hard to advise you on using absolute or relative positioning without seeing what you are doing. It depends on what you want to do and what you're criteria are. If you want to allow someone with 800x600 res to view your website without horizontal scrolling then don't absolute position something that would appear outside of that viewing area.