Block/System/Config/Form/Field/Checkbox.php
public function getCheckedValues()
    {
        if ($this->values === null) {
            $data = $this->config->getImageOptimizationRatios();
            if (!isset($data)) {
                $data = '';
            }
            $this->values = explode(',', $data);
        }
        return $this->values;
    }
public function getCheckedValues()
    {
        if ($this->values === null) {
            $data = $this->config->getImageOptimizationRatios();
            if (!isset($data)) {
                $data = '';
            }
            $this->values = explode(',', $data);
        }
        return $this->values;
    }
Get checked values

@return array|null