If you view the page source the code is
<a onmouseover="Tip('Online Function Grapher written in
JavaScript<br> <br><img
src=../images/grapher.gif width=210 height=175>',SHADOW, true,
BGCOLOR, '#ffffff', FADEIN, 400, FADEOUT, 400)"
href="../grapher/grapher_e.htm">Function-Grapher</a>
Download the javascript file(s) per their instructions and
try it out using your own image.