Update requests for images to use `url`

Currently, dashboard requests images using the image property of an image JSON object and this leads to a mixed-content warnings.

It should use url:

{
    "altText": "",
    "credits": "",
    "height": 899,
    "id": 139,
    "image": "http://dashboard.aura.radio/site_media/images/0fb642b9efb92b2d0b2fb4b0514426d1c0d6f6ac0c7d1f3f3bb57518ed122e06.jpg",
    "licenseId": null,
    "ppoi": "0.5x0.5",
    "thumbnails": [
        {
            "height": 480,
            "url": "/site_media/__sized__/images/0fb642b9efb92b2d0b2fb4b0514426d1c0d6f6ac0c7d1f3f3bb57518ed122e06-crop-c0-5__0-5-640x480-70.jpg",
            "width": 640
        },
        {
            "height": 200,
            "url": "/site_media/__sized__/images/0fb642b9efb92b2d0b2fb4b0514426d1c0d6f6ac0c7d1f3f3bb57518ed122e06-crop-c0-5__0-5-200x200-70.jpg",
            "width": 200
        },
        {
            "height": 150,
            "url": "/site_media/__sized__/images/0fb642b9efb92b2d0b2fb4b0514426d1c0d6f6ac0c7d1f3f3bb57518ed122e06-crop-c0-5__0-5-150x150-70.jpg",
            "width": 150
        }
    ],
    "url": "https://dashboard.aura.radio/site_media/images/0fb642b9efb92b2d0b2fb4b0514426d1c0d6f6ac0c7d1f3f3bb57518ed122e06.jpg",
    "width": 900
}