lightgallery comes with numerous number of beautiful inbuilt animations. You can change type of transition by passing transition name via mode setting. You can easily create your own animations if you have css knowledge.
<div id="fixed-size">
<a href="img/img1.jpg">
<img src="img/thumb1.jpg" />
</a>
<a href="img/img2.jpg">
<img src="img/thumb2.jpg" />
</a>
...
</div>
$('#fixed-size').lightGallery({
mode: 'lg-fade'
})