I just wrote up an article on how to switch an existing WordPress site from HTTP to HTTPS. Basically what’s happening is that you applied that image to your site before you switched to HTTPS. Since the path to the image is stored in the WordPress database, when you applied it, it used your old site address from the general settings before you changed it from http:// to https://, and that’s what’s being pulled from the database when the page renders.
The easiest way to fix this is to go into your appearances -> customize and change the image to something else (briefly) and publish the change. Visit your page and verify the mixed content is no longer being served, then change the image back to your desired image.
From visiting your site, it appears that you have 3 images that are causing this issue. You can read more in the article here. The information you’re looking for will be close to the end of the article under the “Identifying Remaining Issues” heading.