Displaying thumbnail image when rendering PublishingWebControls:RichImageField in edit mode

I have a page layout with the following entry in it:

<PublishingWebControls:RichImageField FieldName="AdvertImage1" runat="server" />

This displays the image when the page is being viewed in display mode, which is fine. In edit mode it displays the standard "Click here to insert an image ... " message which is also fine.

The images are very large, so what I would like to do in edit mode to conserve space is to render a thumbnail of the image to the editor, rather than the image itself. Does anyone know how to do that just by using properties? I can create a custom control to do this, but want to avoid doing so.