2nd and 3rd product images

Hi there.
I know how to control the size of products images for the main image. but how do you control the size of the smaller thumbnails on the product page. You can see the problem I am having here
huniebfashion.com.au/index.php/shop-onli...uct/2671-purple-maxi

Thanks
Nigel

Hi Nicholas
I have updated the code (see below) from 25px to 50 px, created the custom css file for the front end and saved it,but it is not displaying any differently. I have cleared the cache on my browser and refreshed it too

}
div#hikashop_product_left_part img.hikashop_child_image{
margin : 2px;
height:50px;