Hey @Mark_Williams - welcome to the community and thanks for posting.
Based on the second screenshot, it looks like stunnel is using an invalid certificate (eg. locally generated). This is likely causing your browser to block the request for the embedded image since the cert is invalid. This is different from attempting to access the image directly in your browser wherein you can manually accept the invalid cert.
The simplest approach is likely to disable mixed content blocking for SharpTools.io in Chrome using the steps noted in the following article:
For reference, stunnel is most commonly used for secure external access to Blue Iris. As such, it’s more commonly used with a DNS entry that points to your stunnel (Blue Iris) instances external IP Address which can then have a valid SSL cert issued.