When you say original, do you mean the slide compared to the scan, or the first scan compared to the resize for web posting? People have debated this for years, but many prepress/publishing departments will only resize images on multiples of .5 or .25. This is based on the theory that a bitmap image consists of a grid of dots or pixels, and you cannot have a half a pixel, or 1/7 of a pixel, or .78935 of a pixel, either theres one or none. So when you resize at an odd multiple, you can get a severe case of the jaggeds. Just to see, take you original scan and reduce by 50% or 25% only, in photoshop using Image Size>then choosing percent from the drop down menu.