I installed this extension to check it out -- looks like you can't class or ID the tabs -- bummer. So we can't use CSS.
What you'd probably have to do is use JavaScript to insert a new image element inside the tab. I would refer you to the MooTools API, but the code would look something like:
Code:
$$('#mootab_1 li').each( function(item) {
item.inject(new Element('img', { src: 'pathToImg/' + item.getProperty('title') + '.jpg', alt: item.get('text')));
});
I haven't tested this code (I only have a few mins), but in theory it inserts a new image element inside the tab. It's name would match the title tag of tab element (so it's up to you to place and name the images appropriately).
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