Resave all images. After an umbraco Upgrade from 6 to 7 you might have empty .URL's when using the image cropper. Resave all images by calling this API endpoint once.

ImagesApiController.cs

using System.Web.Http;
using System.Collections.Generic;
using Umbraco.Core.Models;
using Umbraco.Web.WebApi;

namespace Our.Umbraco
{
    public class UpgradeHelperController : UmbracoAuthorizedApiController
    {
        [HttpGet()]
        public string ResaveAllImages()
        {
            var mediaService = Services.MediaService;
            var rootmedia = mediaService.GetRootMedia();
            foreach (var media in rootmedia)
            {
                mediaService.Save(media);

                foreach (var item in media.Descendants())
                {
                    mediaService.Save(item);
                }
            }
            return "All media items saved";
        }
    }
}

Really useful, thanks!

I had to rewrite for Umbraco 8: https://gist.github.com/calvindavis/3b6d2572041a62e69179c827557578e0