ssl error

My SSL gaves me a warning so when I check from Chrome’s console I get the error message:
Mixed Content: The page was loaded over HTTPS, but requested an insecure image ‘http://www.britetechs.com/demo/themes/hotelone/wp-content/uploads/2018/09/testimonial.jpg’. This content should also be served over HTTPS.
(index):791 GET https://www.britetechs.com/demo/themes/hotelone/wp-content/uploads/2018/09/testimonial.jpg 404

Any idea in which file is the line that asks for this url?