JQuery :: Parsing Image Columns In Tablesorter

I'm using the tablesorter plugin and having a heck of a time creating a parser to sort a column of images. One of my columns can have one of three images whose HTML code is below:

<img src="/images/intensity_1.png" alt="Mild Intensity" />
<img src="/images/intensity_2.png" alt="Moderate Intensity" />
<img src="/images/intensity_3.png" alt="High Intensity" />

Now using the example of custom parsers from tablesorter's website, I attempted to create my own parser as seen below:

<script type="text/javascript" id="js">
// add parser through the tablesorter addParser method
$.rides.addParser({
// set a unique id

[Code]....

However this code does not work. Every other column is sortable so I know the script itself is working but the column with the images refuses to sort. I also tried using regex to change the images into a sortable number for the script to use but that did not seem to work either.