Category next buttons

Hey everyone I have next and previous buttons on my blog http://www.jokideo.com but if you go to a tab like amazing, the buttons arnt there because if I put them there when you click next it takes you to the next post, not the next post within that category, any ideas to add to this code to make it next post in that certain category? thanks here is my buttons code:

<?php
next_post_link(‘%link’, ‘<img src=”/1_back.png” />’, false);
$random_post = get_posts(‘posts_per_page=1&orderby=rand’);
$link = ‘ID).'” title=”Random Post:’.get_the_title($random_post[0]->ID).'”><img src=”/1_rand.png” />‘;
echo $link;
previous_post_link(‘%link’, ‘<img src=”/1_forw.png” />’, false); ?></h2></div>

Thank you