How to Make an Array Type whose Elements are an Image or URL Type

{
  name: 'referenceList',
  type: 'array',
  title: 'references',
  of: [{
    type: 'url'
  }, {
    type: 'image'
  }]
}

{
  name: 'referenceList',
  title: 'references',
  type: 'array',
  of: [
    {
      type: 'image'
    },
    {
      title: 'URL',
      name: 'urlObject',
      type: 'object',
      fields: [
        {
          title: 'URL',
          name: 'urlField',
          type: 'url'
        }
      ]
    }
  ]
}