SSL not working

I do not display any images (done via CSS)

The site will not display a proper padlock because:

“An image with an insecure url of “https://www.greenhandsgardeningservice.co.uk/wp-content/plugins/testimonial-builder/assets/images/test.png” was loaded on line: 341 of https://www.greenhandsgardeningservice.co.uk/.
This URL will need to be updated to use a secure URL for your padlock to return.”

The page I need help with: [log in to see the link]