In the OS 2 version of Pro Blogger we have introduced the following additional customisation option

This gives the store owner the ability to decide on how they want images to fill out the items in the slider and to get the most out of having standard image dimensions.

In summary:

ContainShrinks down the image to fit the space. Best to use when your images have mixed sizes or you want to ensure customer can see the whole item.
CoverExpands the image to fill/cover the space. Best to use when your images are standard sizes or when clipping part of an image is no issue.

Contain Example:

The entire product is shown but due the irregular shape there is space on the side.

Cover Example:

The same product image has now been expanded to cover the entire square window