1. Open the page
2. Open the chrome developer tools
3. Switch to network tab
4. click on the filter icon (3rd from the left)
5. enter "jpg" in the textbox below
6. scroll down to the viewer on the page
7. open the viewer
8. hit the "clear" button in the developer tools (removes all network requests)
9. zoom to your desired resolution (every zoom click will add a new network request for the page to the network log)
10. single click the entry at the bottom (last request, the resolution you want) a panel is added to the right, under General > Request URL is the server address of the image, something like this:

https://img.yumpu.com/66559632/1/690x978/3-les-nouvelles-28-kundenmagazin-galeries-lafayette-berlin.jpg?quality=80

• the first number is probably the magazine id (66559632)
• the second number is the page number (1)
• the third parameter is the resolution (690x978)
• the remaining url is not relevant (https://img.yumpu.com/66559632/1/690x978/ loads the same image as the full link)

This way you get the basic image url with the desired resolution, you just need to increment the page number (javascript, spreadsheet software, ...) to get all links. you can feed those links to a download manager of your choice.