Nb-user should fetch URL

### Issue type

* [ ] bug report
* [x ] feature request
* [ ] question about the decisions made in the repository

### Issue description

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Expects a local path to the image, however if I am getting images from remote URLs, it should fetch the image and display it. Something like a `[pictureURL]` property on `nb-user` would be very useful.. Id imagine I'm not the only one looking for something like this.