How can I render an image_field from within hook_block_view?
I think a combination of field_get_items() and field_view_value() would do the trick:

$image_items = field_get_items('node', $node, 'field_image');
$image_view = field_view_value('node', $node, 'field_image', array_shift($image_items));

$block['content'] = render($image_view);