Many internet browser disable printing background images and colours by default. This means that when printing certificates from your browser, or saving them as pdf, background images and colours will not be visible.
You can enable background image and colour printing as follows:
Firefox - Firefox > Print > Page Setup > Format & Options> Print Background
IE - Cog > Print > Page setup > Print Background
Chrome - After clicking 'Print' there is a checkbox to enable 'Background colors and images'
Safari - Select File > Print > From the Copies & Pages pop-up menu, choose Safari > Select Print Backgrounds, then click OK