core-bundle/src/Resources/contao/dca/tl_theme.php
public function addPreviewImage($row, $label)
	{
		if ($row['screenshot'] != '')
		{
			$objFile = Contao\FilesModel::findByUuid($row['screenshot']);

			if ($objFile !== null)
			{
				$rootDir = Contao\System::getContainer()->getParameter('kernel.project_dir');
				$label = Contao\Image::getHtml(Contao\System::getContainer()->get('contao.image.image_factory')->create($rootDir . '/' . $objFile->path, array(75, 50, 'center_top'))->getUrl($rootDir), '', 'class="theme_preview"') . ' ' . $label;
			}
		}

		return $label;
	}
public function addPreviewImage($row, $label)
	{
		if ($row['screenshot'] != '')
		{
			$objFile = Contao\FilesModel::findByUuid($row['screenshot']);

			if ($objFile !== null)
			{
				$rootDir = Contao\System::getContainer()->getParameter('kernel.project_dir');
				$label = Contao\Image::getHtml(Contao\System::getContainer()->get('contao.image.image_factory')->create($rootDir . '/' . $objFile->path, array(75, 50, 'center_top'))->getUrl($rootDir), '', 'class="theme_preview"') . ' ' . $label;
			}
		}

		return $label;
	}
Add an image to each record

@param array  $row
@param string $label

@return string