Server-side images don't seem to load

The only really consistently reliable way to include images in email is to convert them to inline Base 64 data. See the following for example: elmah.io/tools/base64-image-encoder/ This means that essentially there is no reference to an external server, and all images in the email are essentially "self-contained" data within the email.

I have considered adding an email tool to do this for emails sent from the FTH websites, but it is a very technical thing that I am not sure how many people would really understand.

I can totally do this.

Why not make it an advanced selection and let the server handle it on the way out? It doesn't have to be entirely user-facing.

Apparently, there is some updated guidance on this... Please take a look at the following:

mailtrap.io/blog/embedding-images-in-htm...e-the-rules-changed/

What are considered good practices for HTML email are often changing.