MarketPress/FrameMarket/Childtheme : how to retrieve cat ID

I'm using Advanced Custom Fields plugin to install a image on the product category top pages (I guess it would be a Category Archive page in plain on WP)

When I use the code :
<img class="category-image"
src="<?php $value = get_field('category-image', 'product_category_24' );
echo $value ; ?>" width="640" height="200" border="0">
It works fine.

But I need to make the "product_category_" ID dynamic to reflect the current category.

So I tried:
<?php $value = get_field('category-image', 'product_category_'.$cat );
echo $value ; ?>
but no luck

I've also tried it replacing $cat with $category_id, but no luck

How can I retrieve the current category ID for this to work?