Google Chrome has flagged a client's Wordpress site as Not Secure. So we are trying to remove a Mixed Content warning that we get using the Inspect feature of the browser.

Mixed Content: The page at '/' was loaded over HTTPS, but requested an insecure image '.png'. This content should also be served over HTTPS.

When we contacted the company who has built the template, they told us to refer to the template code's CSS and located a file in which the following definition was made:

/*********************************************
****************** LOGO **********************
**********************************************/

.site-header .logo {
    position: relative;
    margin:0 auto;
    width: 140px;
    height: 65px;
}

.site-header .logo_image {
    width: 100%;
    height: 100%;
    z-index: 999;
    background: url('../../images/logo.png') no-repeat left;
    background-size: contain;
}

Now, the resource URL that is causing the Mixed Content warning does not have the name logo.png but .png (of course I have changed the real names of the files) as stated in the warning message above.

Moreover, I found a few other files in the CSS code's header definition that contains the logo.png file.

This has not removed the warning.

So my question is, where, in the code can I find the real source of this file that is, for some reason, still under HTTP?

I need to add that we have cleared the cache on both Wordpress and the browser to no avail.