Only the width is defined in K2's parameters.
You can hardcode a height for the container of the image in your CSS file.
In the category.php file you can add a new element and set a fixed height with overflow: hidden.
<?php if($this->params->get('catImage') && $this->category->image): ?>
<div class="categoryImage">
<!-- Category image -->
<img alt="<?php echo K2HelperUtilities::cleanHtml($this->category->name); ?>" src="<?php echo $this->category->image; ?>" style="width:<?php echo $this->params->get('catImageWidth'); ?>px; height:auto;" />
</div>
<?php endif; ?>
Remember to use overrides.
More information on overrides can be found here:
getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates 
