I used to be a real nitpicker when it came to preparing images for the web. I’d laugh at people using large images in web pages, showing them in a smaller format by setting the width and height properties.
In the days before broadband was everywhere it was bad karma to do this, because a large image file would take a long time to download. You needed to prepare the image at the size you were going to be displaying it. Nowadays, things are a little different.
More Browsers are pretty quick at scaling images