Hi Bremen,
I have added thumbnail.png in _diffs/images & it is working fine for me, you can see this in attachments.
And for making classic theme as a parent theme I have added this line in my build.xml
<property name="theme.parent" value="classic" />
Make sure that if you add thumbnail.png in yourtheme/_diffs/images then it should replace thumbnail.png in yourtheme/images and the size & extension should be the same.
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