External images aren't displayed



On my website I offer ad banners that users can implement in their websites, but for some reason they aren’t displayed on these external websites.

This is an example website where the external image should appear at the top (above the headline “Widerruf”):

Only if I would change the url in the inspector of the browser and change it back, it appears.
I have set the file permissions of the image to 755, but also tried it with 777 and it didn’t work.

Do you have an idea why?

Thank you.