There are probably better ways of doing this but I'm not a very good coder and I avoid it as much as possible. My problem is fixed now, I think.
Anyway, the plugin draws information like this:
1) featured image
2) first image on page
3) default image (which you can set, eg logo of your website)
You may be able to disable the second one, so only the first and third option work. You could contact the developer or browse the support page.
Does your theme support featured images? Otherwise this plugin won't work for you anyway.