How to download a large page image from an Issuu magazine:
1. Load the Issuu magazine.
2. Right click on the first page. “View Source”.
3. Near the top of the source code, look in the meta for
link rel="image_src" href="http://image.issuu.com/[really-really-long-number---]/jpg/page_1.jpg"
4. Copy that image URL, and simply change page_1.jpg to page_18.jpg or whatever page you want to get.
Note that the page numbering between the Issuu magazine’s display and the original PDF may be astray by a few pages, and a little re-calibration may be needed. Note also that you’re getting images of the entire page, not hi-res copies of the images used in the page.
But these days all this is probably not needed. Screen-capture software is perfectly capable of capturing a scrolling screen.