Show only one category

CHRISTMAS EDITION

Due to Christmas time and my brain hitting some serious technical difficulties, I got get_the_category() and get_the_categories filter mixed up.

Here is the revised filter function, working:

You can also make use of the get_the_categories filter

add_filter( 'get_the_categories', function ( $categories )
{
    // Only return the first object in the array
    return $cat[] = $categories[0];
}); 

ORIGINAL ANSWER

Because you pass everything directly to your foreach loop, the easiest ist to break the loop after the first iteration

<?php 
foreach((get_the_category()) as $cat)
{
    $catname =$cat->category_nicename;
    echo "<a href=\"/category/";
    echo $catname;
    echo "/\">";
    echo "<img src=\"/wp-content/categorias/";
    echo $catname;
    echo ".png\" alt=\"$catname category image\" /></a>\n";

    // Lets break the loop after one iteration
    break;
}
?>