Solution:
That’s good that you are sure about not any image is on http. Yet you are facing problem of non secure warning in the browser so you should try some methods listed below to check your website for http path (if any):
Look for the unsecured contents
With plugin find out unsecured (non-https) content
Use trusted third third-party website for searching out the mixed stuffs
Checkout resource based on above three points in detail to solve mixed content warning.