if is tag

i need to apply a image, if a post has a certain tag on it.
i just check de codex and there is a function about that, wich it is:

is_tag( 'tagname' )

but my problem is, that i dont know much about php, so i wrote this:

<?php if ( ! is_tag( 'tagname' ) ) {?>
     <div class="bullet-pronto"><img src="image.jpg"></div>
<?php } ?>

So, i need to detect, if there is a certain and specific tag, show an image, but that code is note working for me, any idea?