Retrieve the URI of an image field, D8

I feel suddenly dumb… I’ve not gone through with this today: get the URI of an image field of a custom block.

The documentation and the stackoverflow issues talk about content.field_id.entity.uri to get the URI of an image field.

Sounds stupid, right? But how do you do to get the URI for an image field ? Because in my case, D 8.2, there is not any URI (I use kint, plus Devel). I’d like to use file_url() to render the field as a background.

Regards.

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