Unlike screenshots, the background of a torrent description page can not usually be right-clicked and saved, due to the mouse interacting with the frame. It is possible to locate its URL and open it in a separate tab and THEN save it though, if you can find it in the source code.
If using Chrome, Press F12 then Resources->Frames->(xxxxxxx-torrent-title-group.html)->description_iframe->images
- Right click on the image and click 'View Frame Source'. Make sure not to click 'View Page Source' by mistake.
- Search for 'background-image' and highlight the URL to the right of it and open it in a new tab.