You can change the size of your thumbnail pictures this way:
1) Log into your store admin
2) Click "Catalog Setup"
3) Change the width or height of your thumbnails
Note: If you set a fixed width AND height for your thumbnails and your thumbnails are not that exact size, it will cause some distortion. We recommend choosing one or the other (width or height) and leaving the other blank. This will avoid picture distortion. Also, your thumbnails will look best and most professional if your actual thumbnails are the same size as the width or height you choose in your store admin.
