dpi settings are irrelevant on the web and will be ignored by web browsers, which will display your images at 1:1 pixels. Xtreme may internally think that a pixel is 1/96 of an inch, but that information is meaningless when it comes to exporting a web page.

if i buy some pictures @ 300 dpi does xara change them to 72 dpi or 96 dpi?
Xtreme exports images at whatever pixel size you see when you drop them on your page. When exporting for web, it tries not to give any dpi information, as it's irrelevant. (ie. no pHYs chunk in PNG.)

As Jimi said, typically the use of having bigger/higher-res pictures is so that you can resize and crop them to fit the size you want. Smaller images tend to look poor/blurry/deformed when you try to scale them, so if you've got a little image you've nicked off a web page, you are pretty much stuck with that size.