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Rememnber that unless you change the filesize of the pic, the difference in loading with be very very very minimal or almost no difference at all. =)
One is when you are using the same image at more than one size on a page. Then it makes sense to download the image once (a file at the largest resolution you need) and use the browser to resize the same image for the smaller versions.
Another time is when you have an image file that will not pixelate -- because everything is at right angles (or even just a solid color). Then it makes sense to create a small version of the image and size up to a larger rendering using the browser dimensions.