change thumbnail size for certain category

Hello,

I did some css modifications on my website, so that a certain category is displayed different on the frontpage (smaller image, smaller font, etc).

Here’s what I added:

.category-linkdump .post-title {
    font-size: 18px;
    text-align: right;
}

.category-linkdump .post-thumbnail {
    width: 30%;
}

Now, the website resizes the thumbnails for that category from 520×292 to 134×75.

But my theme provides a 190×106 version of every image, so I want to use that one for the specific category.

This is what I did (and it didn’t work, that’s why I’m here). I changed:

<?php if ( has_post_thumbnail() ): ?>
<?php the_post_thumbnail('split-medium'); ?>
<?php elseif ( ot_get_option('placeholder') != 'off' ): ?>
<img src="<?php echo get_template_directory_uri(); ?>/img/thumb-medium.png" alt="<?php the_title(); ?>" />
<?php endif; ?>

to:

<?php if is_category( 'linkdump' ): ?>
<?php the_post_thumbnail('thumbnail'); ?>
<?php elseif is_category( 'Stof' ): ?> 
<?php the_post_thumbnail('split-medium'); ?>  
<?php endif; ?> 

What did I do wrong? My theme is ‘Split’ by Alexander Agnarson.

Hope anyone can help!