Updating image with `foaf:picture` in context - lost after truncated to Compact IRI syntax

See components/solid-directory!81 (comment 51882) for background

The image isn't updated because PyLD (in JSONLDParser) replaces picture in the POST body with foaf:depiction, truncating the request like so

{'@context': 'https://cdn.happy-dev.fr/owl/hdcontext.jsonld',
 '@id': 'http://localhost:8000/accounts/test/',
 'slug': 'test',
 'foaf:depiction': 'https://github.com/calummackervoy/calummackervoy.github.io/blob/master/assets/img/profile.jpg'}

Notice the foaf:depiction key in the compacted data. In JSON-LD syntax this is the Compact IRI, where picture is the default serialization when all fields are in the same vocabulary

All of our fields in the context are not in the same vocabulary, though, so I'm not sure why name is serialized in the default syntax even though the context passes it as 'name': 'rdfs:label', it seems inconsistent

Anyway the outcome is that the serializer receives foaf:depiction and has no idea that you're talking about picture, because fields aren't linked to URI properties in DjangoLDP. The field is ignored and the picture isn't updated