At the moment category images can be used in a few different ways using special tags and code in a page template...
{slider,categoryid,width,height} - this will display a dynamic image slider that will use images from the category specified and you can specify the width and height of the image slider.
The following php function can be added to a page template as well, it displays a table that has the main image of each category in each cell and these images automatically link through to the corresponding category.
<?php
// display a table of main category images with links
$category_ids = array('1','2','3'); // an array of category ids
$cols = 4; // the number of table columns
$tnx = 150; // image thumbnail width
$tny = 150; // image thumbnail height
echo display_category_image_table($category_ids,$cols,$tnx,$tny);
?>
In the future category images will probably be used for image galleries as well but this hasn't been added yet.