Notice about insecure content varies from browser to browser. Example of a Firefox notice:

 Connection Is Not Secure. Parts of this page are not secure (such as images)  

• Received as: notification in browser.
• Happens after: SSL installation
• Caused by: non secure (http) content being loaded on a secure (https) page.
• Solution:
  • open Firebug Console or Chrome Developer Tools.
  • refresh the page with enabled Firebug Console:
  • detect “Loading mixed (insecure) display content” console messages:
  • select and copy URLs of insecure content one by one
  • open copied URLs in separate browser tabs to view the content
  • re-upload insecure content.

Theme related images are: logo, background image and header image. If you see theme related image after you open copied URL in browser tab:

• go to Admin Panel » Appearance » Customize » Theme Settings.
• re-upload all images that were listed in Firebug's/ Chrome Developer Tools' console.
• re-check page load with enabled enabled Firebug Console.

Troubleshooting requires active support service.

• activate support service ( if inactive)
• create support ticket with admin panel and FTP access details.