I'm trying to show a thumbnail in summary gridfield.

static $has_many = array(	'Images' => 'Image');

static $summary_fields = array(
		'Title' => 'Title',
		'Thumb' => 'Thumb'
);

public function getThumb()
{
	if($this->Images()->count())
        {
			$image = $this->Images()->first();
			return $image->CMSThumbnail();
	}
	else return "No images.";	
}

$image contains the correct Image object, but CMSThumbnail returns null, why?

Maybe I found the problem, but i can't solve it.

$image->Filename returns assets/{imageName}

Images are uploaded in 'assets/upload', but Silverstripe looks for them in assets. Why?

sorry I did not read the documentation carefully, the problem is due to relation 'has_many' for images. Changed to 'many_many' and seems to work properly.