This is possible in v2.0 Email Templates.Define it as follows and the "alt" attribute will be retained on the <img> even after the user selects one in the Email Editor.<div class="...
New email editor will show you what Snippet is being used in the email. We will probably enhance the Email Summary Page to also show info like this in the future
We've done extensive evaluations of what we would build and how. Right now, it's not on the short term roadmap. Other features have been prioritized higher.