That'd be right clicking to snatch the chart URL, which contains the stock symbol and date parameters. Peeking into the future would certainly help one's rate of return a bit :)
<?php header("Location: http://quote-web.aol.com/..."); ?>
Then you don't have to use image processing (including actually reading the image), and they'd have to become right-click-and-copy-paste-image-src-check-in-new-tab-cheater! :-)