I think it depends on the software's targeting user group. This is okay, and probably the preferred behavior if your users are all tech-savvy. But it is hard to explain to non-technical users why this ugly text email is better than that that email with beautiful pictures, or even what HTML is.
The pictures aren't in the email. The email contains instructions saying “phone Steve and ask for the images, then put them in this gap”, but if your computer follows those instructions then Steve knows when you're reading your emails, and where.
Who is Steve? Nobody knows, but he's in the “knowing who's reading emails and when” business. It's a shady business. Don't let your computer phone Steve.
Turning in html should be an option done only when really needed.