Please remember that the product image files you upload have maximum height and width. In case you upload images with other dimensions or different aspect ratio, your images may get distorted.
In order this to be changed, please log in the administrative interface of your ZenCart and open the Configuration menu. Select the Images option.
By selecting the image settings for the different page types and then clicking the Edit button you can update the dimensions.
Setting one of the two dimensions to zero will make the images resize according to their own aspect ratio, rather than to the pre-defined one.
Please, visit Managing the Product Catalog for detailed information about the products setup.