That page throws 2 mixed content warnings (one warning, for your JPG and one fatal, for an external script). This page won't get a padlock symbol, due to this.
Now, why your image is fatal on diyAudio and not a warning, I'm not sure just now. It's possibly because of the highslide script which is loading the image via a script, which is a worse category of issue. But you can be sure of one thing - the situation won't get better over time as browsers ratchet security ever tighter.
Actually I think the problem is on AA, the HTTP version is being loaded. The SSL version isn't, so the cert isn't being checked. On diyAudio, the HTTPS version is attempted, and since the cert is invalid, it's failing. The solution is to get a valid cert, which is free these days:
Let's Encrypt - Free SSL/TLS Certificates.
However don't let people tell you "oh Let's Encrypt is so easy for everyone"... yes, for most it's easy. For some setups like diyAudio's, it was a PITA to get working. There are plenty of keyboard jockeys with a Wordpress site that think they know everything.
Long story short - it's the future, and we need to move on, and you need to get a valid cert. Maybe try using
Cloudflare (free) for your DNS, use their caching services (free) to take load off your server and give you a SSL proxy (free), and you won't have to do a thing. They even have a SSL proxy mode where they ignore the cert validity of the origin (your site) which would work great for you without any changes at all.