Checking the Console.app logs and it appears that Jared is receiving my POST command but the photo never actually sends. Are you using this on Ventura? Possible that an update is required for the latest OS?

{"body":{"message":"Demo"},"attachments":[{"filePath":"~/Pictures/ab.jpg"}],"recipient":{"handle":"REDACTED"}} 

When trying the same type of command with imessage-ruby it has the same issue.