Ok, dumb question, the images on the page are all 72, so you can't have 300. But... what's the best way to print in as high resolution as possible on a brochure?
How about zooming in on the web-page before taking the screen-shot? Most modern browsers allow you to set a custom zoom level.
Note though that the images may not look as great as you would like. You can't do much about this without access to high-res version of the images.