Petra,
That is most unusual. I’ve just recently switched web hosts and have noticed in Safari on my demo, the featured image does not show, but it does in Firefox and Chrome.
On your site, I can find the image in Firefox and Chrome (on my Mac) but not in Safari.
What’s even stranger is that on my local copy of the theme, I can see the featured image in all browsers.
I did not encounter that on my old host. I wonder if it’s somehow server related. If you don’t mind me asking, who do you use for your web hosting?
I’ll dig into this today and try to find an answer for both of us.
Sorry,
Bill