Images can be exported only if the browser is able to load them during exporting via Ajax, or if CORS is allowed (which usually isn't). The following documentation section explains this:
In this particular case, the web server, which hosts the image, does not allow cross-domain Ajax requests and you will see an error in the browser console during export:
Image from origin 'https://static.pexels.com' has been blocked from loading by Cross-Origin Resource Sharing policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://runner.telerik.io' is therefore not allowed access.
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