Hi,

It's very easy. In webforms you just create a user control and inherit from ContentControlBase, just like you would do for content areas:

    [TemplateDescriptor(Inherited = true)]
    public partial class ImageControl : ContentControlBase
    {
        protected Injected UrlResolver { get; set; }

        protected string ImageUrl
        {
            get
            {
                return UrlResolver.Service.GetUrl(CurrentData.ContentLink);
            }
        }
    }

One warning though, the image will be rendered as a block element in the editor, so it's not possible to have it floating next to the text.

For future reference this is how it is done in MVC when dropping a page in the editor.

   [UIDescriptorRegistration]
    public class PageBaseDescriptor : UIDescriptor<PageData>, IEditorDropBehavior
    {
        public EditorDropBehavior EditorDropBehaviour { get; set; }

        public PageBaseDescriptor()
        {
            EditorDropBehaviour = EditorDropBehavior.CreateContentBlock;
        }
    }