Is This Mixed Content Issue Fixed?

Hey there,

Junior SEO here so go easy! I've recently come across a mixed content issue with one of our clients - HTTP img files in the DOM. However, when I visit said unsecured URL, it automatically 307 redirects me to an equivalent secured resource.

Is this an issue at all? From what I've read this means they've implemented a HSTS and all should be right with the world? Is there any step they've missed that I need to flag here?

Any and all help appreciated and welcome :) Thank you in advance!