Inside the index.php of the theme, you have some html that looks something like this:
<li><img class="active" src="<?php bloginfo('template_url'); ?>/images/img04.jpg" title="Monument Valley" alt="" width="604" height="375" onmouseover="this.style.opacity=1;this.filters.alpha.opacity=100"
onmouseout="this.style.opacity=0.4;this.filters.alpha.opacity=70" /></li>
<li><img src="<?php bloginfo('template_url'); ?>/images/img05.jpg" title="Honey Bee" alt="" width="100" height="75" onmouseover="this.style.opacity=1;this.filters.alpha.opacity=100"
onmouseout="this.style.opacity=0.4;this.filters.alpha.opacity=70" /></li>
<li><img src="<?php bloginfo('template_url'); ?>/images/img06.jpg" title="Cup of Coffee" alt="" width="100" height="75" onmouseover="this.style.opacity=1;this.filters.alpha.opacity=100"
onmouseout="this.style.opacity=0.4;this.filters.alpha.opacity=70" /></li>
<li><img src="<?php bloginfo('template_url'); ?>/images/img07.jpg" title="Grand Tetons" alt="" width="100" height="75" onmouseover="this.style.opacity=1;this.filters.alpha.opacity=100"
onmouseout="this.style.opacity=0.4;this.filters.alpha.opacity=70" /></li>
The images are located in the images folder. If you wanted the images in the template to reflect your own custom images, you’d use something like:
<li><img src="<?php bloginfo('template_url'); ?>/your/image/path" title="Your Image" alt="" width="100" height="75" onmouseover="this.style.opacity=1;this.filters.alpha.opacity=100"
onmouseout="this.style.opacity=0.4;this.filters.alpha.opacity=70" /></li>
You can also use WordPress’ built in gallery shortcode [gallery] and style it the way you’d like. Hope it helps.