How do image thumbnails work?

Hi there,

Sorry for the rather vauge topic title but I'm a little confused as to how image thumbnails work in Hikashop. I want to increase the size of main image displayed on the product show page, but when I do so using CSS, it goes all pixlated. I reckon this is to do with the fact that the image displayed is not the full image but some specific thumbnail version that Hikashop has created? I can't for the life of me work out how the "thumnail" system works in hikashop, is there a piece of documentation that describes which thumbnails are created, when there created and how to edit them? If not, could someone tell me the answer to these questions and any other useful information they may happen to know on the topic?

Many thanks,

Jonathan