Easy... The world of email is not monolithic...
(1) It depends on the client
(2) It depends on the user settings within the client
(3) It depends on the server
(4) It depends on the user settings on the server
I did state that the image solution is not reliable but it works - when it does. And if it never didn't, as we're led to believe, then why were these tools developed? And why are linked images still being sent?
Having said all that, one must remember that there are laws against spamming and if that's what the OP intends to do, then good luck...
Of course, if it is spam, the linked images are not likely to reach the intended end-reader and the issue becomes moot anyways.
Just last night - to some textent verifiable because I commented it in one of the threads - I sent out about 50 inivitations for a dinner party - HTML + linked graphics included. I realize that it is a very, very empirical sample but, nonetheless, everyone has received AND opened the mailed invitations, including graphics.
It is in the nature of forum posts to be definite and ultimative in the statements we make - note the "we", no one exempted here - but each PC is different, as is each user, settings vary across the spectrum, and internet knowlege goes from nil to expert.
I find it dangerous to say "it won't work anyways" because someone else just reading may come to believe that it's true and will perhaps be a little less careful based on such well-intended but nonetheless less erroneous statements. We'd like it to be that way, but, unfortunately, it isn't.
Or why are the Nigerian scams still "working"?