After quite some Google searching I found some answers that came close, but nothing that worked straight away. So to prevent you from puzzling and searching, here is how to get the image field url from a referenced node in a Twig node template
Use this code for example in node--blog--teaser.html.twig :
<img src="{{ file_url(node.field_REFERENCE.entity.field_IMAGE.entity.uri.value) }}"
alt="{{ node.field_REFERENCE.entity.field_IMAGE.entity.alt.value }}" />
Replace 'field_REFERENCE' with your node reference field system name, and replace 'field_IMAGE' with the system name of your image field.
And by the way: more on Twig file naming conventions in Drupal here.