The image for your Facebook Page will display a question mark (?) if you have age restrictions on the Facebook Page as Facebook prevents us from pulling through the image in this case.

If you want the image to be displayed, please review and adjust the age restrictions for that page to be public as shown in our step-by-step guide here: Checking for Age/Country Restrictions on your Facebook New Page Experience Page 

There's also some documentation from Facebook themselves about this issue here: I can't see my Page when I'm logged out of Facebook.