To expand on Borvik's reply. In CSS, space means something. Likewise it means something in HTML class attribute as well.
In HTML class attribute, space is used to apply more than one class name to the element. Like Borvik said, class="view-field view-data-image-nid" would mean both .view-field and .view-data-image-nid are applied to the element.
In CSS, space is used for descendant selector. So, if you would put the thing in a way you did (.view-field view-data-image-nid), it would mean the following:
Select an element called 'view-data-image-nid' that is a descendant of an element with a class names 'view-field'. Because there is no element with the name like above and because that element would also not be inside the element with the above class, your declaration will never get applied.