This isn't DRM. There's no need to turn text into an image here. I think people are confusing this for some sort of document protection scheme which is trying to do something that's impossible.
You're mistakenly treating the recipient as a hostile entity. If you can't trust them to view and *not attempt to copy* confidential information, then they shouldn't be allowed to see it anyhow.
Instead, you need to consider the recipient as an imbecile who will, for example, accidentally forward a confidential note to their entire address book, or forget to delete the message as requested.
Good security is often about simply making the default behavior secure. In this case, you're simply ensuring that the recipient has to do nothing at all for that information to remain secure, which is about as good as it's going to get once you release information to another party.