We have all been to sites where they use various techniques to try to avoid bots using the service intended for human usage (for example Yahoo mail doesn't want spammers to setup bots to send out e-mail). In many of those tests, there is an image that is used - usually of alphanumeric characters that the user then needs to put into a form. The automated bots have started searching the code on the page for the image, then doing what is essentially OCR on the image and then using that data to submit the form.
I have written a way around that which uses only HTML and CSS to represent an image - but if you look at the source of the page, there is no image and no sign of the text that is seen on screen. This is done by treating DIVs like pixels and recreating the image that way.
I have a proof of concept page up here with a static example, a dynamic example (looking at the source code of either of those pages you will see no images and no text that matches what is on screen), and Perl source code for each (static and dynamic).
This technique could also be used on web pages for obfuscating e-mail addresses since the bots can't scan the source to pull out the text of the e-mail address.
<code> <a> <b> <big> <blockquote> <br /> <dd> <dl> <dt> <em> <font> <h1> <h2> <h3> <h4> <h5> <h6> <hr /> <i> <li> <nbsp> <ol> <p> <small> <strike> <strong> <sub> <sup> <table> <td> <th> <tr> <tt> <u> <ul>