In Umbraco 8.02 I have some trouble getting an image with GetCropUrl. I have a macro from a Umbraco template and the content type has a crop image. Listing the nodes is no problem, just the right crop url. Below is the code I use with some comments. I tried to convert the field value (to what?) and call Url.GetCropUrl. All lead to an empty value or an error message. Someone is able to help me with this?

@inherits Umbraco.Web.Macros.PartialViewMacroPage
@using Umbraco.Web

@* Get the starting page *@
var startNode = Umbraco.Content(startNodeId);
var selection = startNode.Children.Where(x => x.IsVisible()).ToArray();

if (selection.Length > 0)
{

<div class="row">
    @foreach (var item in selection)
    {
    var image = item.Value<string>("headerImage"); // string returns json, without type it returns Umbraco.Core.PropertyEditors.ValueConverters.ImageCropperValue
        <div class="col-md-4">
            @image -> Image url
            @image.GetCropUrl("overview") -> error
            @Url.GetCropUrl("headerImage", "overview") -> error
        <p><a href="@item.Url">@item.Name</a></p>
        </div>
    }
</div>
}