core-bundle/src/Resources/contao/modules/ModuleRandomImage.php
public function generate()
	{
		$this->multiSRC = StringUtil::deserialize($this->multiSRC);

		if (empty($this->multiSRC) || !\is_array($this->multiSRC))
		{
			return '';
		}

		$this->objFiles = FilesModel::findMultipleByUuids($this->multiSRC);

		if ($this->objFiles === null)
		{
			return '';
		}

		return parent::generate();
	}
public function generate()
	{
		$this->multiSRC = StringUtil::deserialize($this->multiSRC);

		if (empty($this->multiSRC) || !\is_array($this->multiSRC))
		{
			return '';
		}

		$this->objFiles = FilesModel::findMultipleByUuids($this->multiSRC);

		if ($this->objFiles === null)
		{
			return '';
		}

		return parent::generate();
	}
Check the source folder

@return string