When to dump a friend ...

[french frank]

I'm not sure that is entirely correct. As I understand it, when an e-mail includes images those images are downloaded separately when an e-mail is viewed (and an image be the size of a dot). That downloading informs the sender that your e-mail address is valid and active - not good for spam or malicious, etc, etc.

Yes, probably correct. I use the ordinary Mac Mail and don't often bother to load the images.

The emails you open unawares are those where you recognise the name of the sender. Does the AV program on a PC not warn of a dodgy image?