Since web crawlers are currently unable to look inside images to scrape email address this means that your email address will not be scraped from a web site, but can be written down by a human.
Here's an example email address:
Made using jeaig
and the code that generates that is:
<font size="-2">Made using <a href="http://jeaig.org/">jeaig</a></font>
The server does not store the email address: when a user enters an email address on the site it is padded with a random amount of random data (both before and after the address) with randomness being supplied by /dev/urandom. Then the address is encrypted using Blowfish using a secret key known only to the jeaig server (this key was also generated from /dev/urandom) and a random IV is chosen.
The encrypted data is then modified base-64 encoded so that it can be used in the URL.
When the image is requested using the special base-64 filename the email address is decrypted and then rendered using CAPTCHA code to produce the image. The image changes each time the email address is loaded.
And, yes, this is a free service.