I think that the fields available for extending modules are in this namespace "Telerik.Sitefinity.Web.UI.Fields". You can find the "ImageField" class there, but if you try to add it it throws exception saying that the "FieldDefinitionElement" attribute is missing. Also, in the control template "Telerik.Sitefinity.Resources.Templates.Fields.ImageField.ascx" there is "alert('In process of implementation.')". So, I guess the field is not complete. I need to add image and video selector for some of the existing modules like you and I will welcome a solution from Telerik instead of taking the hard and painful path of workarounds.