Upload a LinkedIn image

Requires scope: `publish:write`. Uploads a base64 image and returns an `asset_urn` for use in a post's metadata. Asset URNs are scoped to the uploading account — pass `social_account_id` when the user has more than one LinkedIn account connected.

curl -X POST "https://example.com/linkedin/image_uploads" \  -H "Content-Type: application/json" \  -d '{    "content_type": "string"  }'

{  "asset_urn": "urn:li:image:abc123"}