Change image link throwing SSL error

Hi

A website im working on is throwing a mixed media SSL cert error because of an image below the left hand side navigation menu (the tick image in the call me back button), the link to the image is using https:// and needs to be changed to https:// but I cannot find where the image is link is located, it isnt in the theme style.css sidebar.php, single.php or page.php file , where else might it be located?

Help appreciated

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