With this Behat function you can create a screenshot when a test fails.

makeScreenshotOnFailure

<?php
    /**
     * @AfterStep
     */
    public function makeScreenshotOnFailure($event)
    {
        if (StepEvent::FAILED !== $event->getResult() || !($this->getSession()->getDriver() instanceof Selenium2Driver)) {
            return;
        }

        $session = $this->getSession();
        $step = $event->getStep();
        $fileName = $step->getParent()->getTitle().'.'.$step->getType().' '.$step->getText();
        $fileName = preg_replace('/[^a-zA-Z0-9-_\.]/', '_', $fileName).'.png';
        $screenshotsDir = $this->kernel->getRootDir().'/../screenshots/';

        if (!file_exists($screenshotsDir)) {
            mkdir($screenshotsDir);
        }

        file_put_contents($screenshotsDir.$fileName, $session->getScreenshot());
    }