What I use for this is freeware EasyThumbnails, you can find it here:
http://www.fookes.com/ezthumbs/
Using this I copied your "Honey Coloured Braclet" pic that is natively 2304X1536 pixels with a size of 351KB and converted it to the Zen size of 120X80 pixels resulting in a size of 1.5KB.

Zen allows for three sizes of pics that are all in the images directory at the root and also in the large and medium sub dirs. These relate to the standard image used for standard display, a medium image for when a product description is loaded, and a large image when a customer clicks 'larger image'.

The smaller image sizes load quicker and due to smaller size do not have to suffer in clarity. This provides your customers with faster loading pages. If the customer is interested, they might want to view a larger image and are interested enough to allow the larger time to load.

You should match your image thumbs to the sizes specified in your admin/config/images.

Easy Thumbnails allows batch conversion so that this is not a tedious image at a time process. Just convert them, FTP to their respective dir's and ZEN will load the correct image size.