Exporting unnecessarily hi-res images is really bad for website performance, so Web Designer tries to avoid this, and always produces, as standard, 96dpi images which is the official standard resolution for web browsers (W3C CSS pixel definition is 96dpi). So if you place a hi-res image on the page (or scale an image down) to be higher than 96dpi (the status line in Web Designer shows you the resolution of the image on the page), then it's re-generated at 96dpi. This produces the fastest possible download performance for your website - and all graphics will look exactly the same on the website at 100% as they do in Web Designers at 100% view.

However if you have the 'Retina (hidef) images' option on (in the Web Properties dialog, Website tab), then we will also create double resolution versions (192dpi) of all graphics and images, which is designed for hi-res screens of modern phones, tablets and computers. This means all images and graphics are pin-sharp on hi-res screens. If you zoom, I believe desktop browsers will switch to using the hi-res one. That seems to be the case in my tests. So try that option.