Associate category with images

I would like to have an image related to the category to place it in addition to the “filed in” information.
I’m inserting the <?php the_category_ID() ?> tag as advised in the doc, but it only gives ‘0’ in reply. :'(
I was trying to insert the image by calling the ID.gif with the following line :
<img src=”/images/<?php echo the_category_ID() ?>.gif” />
I get 0.gif file for every post… :-/
I’m totally 0 also in php ! 🙂 Anyone could help ? (please !)