How to eliminate an error link (http) to a not existing photo in header area

Before my site had https security I had uploaded a photo to the header area which I already eliminated from the media files several months ago. The source code still shows me a url-http-link to this photo. It makes my site slow because it's in the header area and is uploaded with every page. Furthermore I am receiving the message from Google that my site is not secure because of some photos. I assume that this photo is the major problem.

How can I eliminate this http-error link (404) as shown when I scan the site?
Do I need to access source code? And how do I do that without breaking my site?

My host told me I need to contact the theme-developer for that issue, probably because the photo was uploaded in the header area.
This is what the website checking tool says:

Mixed Content: The page at 'https://beste-vpn-anbieter.com/' was loaded over HTTPS, but requested an insecure image 'http://beste-vpn-anbieter.com/wp-content/uploads/2016/02/VPNLogo1.png'. This content should also be served over HTTPS.
(index):250 Uncaught ReferenceError: add_filter is not defined
at (index):250
contentscripts.js:297 init
/wp-content/uploads/2016/02/VPNLogo1.png Failed to load resource: the server responded with a status of 404 (Not Found)

So I assume I need to eliminate the VPNLogo1.png photo link from the source code.

If you could help me with information on that, it would be awesome.
Thank you so much.