Given you have an URL property with a UI hint to indicate that the editor should select an image, like this:
[UIHint(UIHint.Image)] public virtual Url TopImage { get; set; }
Then you can make it render as an image by placing a partial view in the /Views/DisplayTemplates folder named Image.cshtml with the following content:
@model EPiServer.Url @if (Model != null && !Model.IsEmpty()) {<img src="@Model.ToString()" alt=""/> }