This would be a <picture srcset>
but we do not know how widely this would be supported.
See also example 51 in the Spec.
{
"@context": "https://www.w3.org/ns/activitystreams",
"type": "Image",
"name": "Cat Jumping on Wagon",
"url": [
{
"type": "Link",
"href": "http://example.org/image.jpeg",
"mediaType": "image/jpeg",
"width": 400
},
{
"type": "Link",
"href": "http://example.org/image.png",
"mediaType": "image/jpeg",
"width": 1920
}
]
}