You can do something like
<?php
$current_category = get_the_category();
/*
$current_category[0]->ID is holding the current category now.
If there are more than 1 categories for the post you should iterate
through all of them and check if the current category you're
displaying is in there
*/
$cats = get_categories('hide_empty=0&child_of=2&orderby=count&number=99&order=asc');
foreach ((array)$cats as $cat) {
$catdesc = $cat->category_description;
?>
<li class="<?php echo ( ( $current_category[0]->ID === $cat->ID )? 'current' : '' ); ?>">
<a href="https://wordpress.stackexchange.com/questions/131081/<?php echo get_category_link($cat); ?>" title="<?php echo strip_tags($catdesc); ?>">
<img src="<?php echo get_home_url (); ?>/wp-content/uploads/<?php echo $cat->slug ?>" alt="<?php echo $cat->cat_name; ?>" class="front-img">
</a>
</li>
<?php } ?>