Emails are not protected. If an attacker is specifically against you and has technical read access to your emails, then he will be able to open pictures and see their contents. Encoding text as pictures is good only to deter robots. This uses the known facts that although human brains are, as far as computing goes, remarkably feeble, they are outstandingly good at image recognition. That's the reason we write with, say, writing: our brains are easily trained to recognize contiguous ink blots as letters (or other glyphs), filtering out visual noise automatically.
CAPTCHA systems use the same principle. They try to defeat automatic scanners. In practice they don't work so well, in part because attackers have learned to hire cheap human labour to do the image recognition part.
An encrypted Zip is something really different. This is cryptography. It does not work on the hardness of recognizing letters within a picture; it works on the hardness of breaking an encryption algorithm if the key is not known. An encrypted Zip will deter attackers, and prevent even targeted, human-controlled eavesdropping; but it will do so only if the decryption key (the archive "password") can be conveyed to the intended recipient without showing it to the attacker. Do NOT write the Zip password in the email itself (don't laugh, I have seen it done -- you may cry, though; that's what I did). A Zip archive protected with a password that you give to the recipient over a phone call ought to be effective against most attackers (if the bad guy taps on your emails and on your phone, then you probably have bigger worries).