Aram,
First, make sure you're on the latest DOM revision, because I remember fixing a bug related to this a few weeks ago, but I can't say whether it was exactly this, because AFAIK this has always worked.
Make sure your custom getter is in the format get{FieldName}. That seems to be the error in your code. My guess is that getThumbnail() will return the intended effect. Does for me, anyway.