When specifying element style icons or the branding logo as a URL, if you are using the cloud service, the logo must be served over HTTPS.
Also, for security reasons (see Allowing cross-origin use of images and canvas for more details),
the web server hosting your image must set the
Access-Control-Allow-Origin header to allow cross-origin requests.
Failure to have this header will mean that the process of exporting your diagrams to PNG will fail.
To avoid these problems, you can instead specify your icons as a base64 data URI instead. You can find utilities to convert an image file to a base64 encoded data URI in the client libraries, and online.