Comment 4 for bug 1258418

[
{'url': 'http://example.com/media/catalog/product/a/f/afmetingen_picture.jpg',
'label': 'Afmetingen picture',
'file': '/a/f/afmetingen_picture.jpg',
'position': '2'
, 'exclude': '0'
, 'types': []}

, {'url': 'http://example.com/media/catalog/product/0/1/verschil_picture.jpg'
, 'label': 'Verschil picture'
, 'file': '/0/1/verschil_picture.jpg'
, 'position': '3'
, 'exclude': '0'
, 'types': ['image']}

, {'url': 'http://example.com/media/catalog/product/0/1/standard_picture.jpg'
, 'label': 'Standard_picture'
, 'file': '/0/1/standard_picture.jpg'
, 'position': '4'
, 'exclude': '1'
, 'types': []}]

It seems that 'types': ['image'] is the image we are after. This one has the checkbox "Base Image" set.

I should have mentioned we use Magento 1.4.1.1.