How does a valid logo URL in the style area of a satisfaction survey look like?

Hi experts,

how does a "valid" logo URL in the style area of a satisfaction survey look like?

I inserted a link that works, if I paste it in a new browser tab, but in the style area for surveys in C4C I got an error message while pressing "Apply"... Although I can see the picture in the sample area.

The URL inserted is with https protocol and the picture file is in png format.

Thanks for help!

Best regards,

Deborah