There are 2 approach on this:
1.) Resize the images first before uploading the images to make sure the images have the same height. The ideal size for this is 200px width and 140px in height.
2.) You can upload different image sizes, however, it might not look good and might show only part of the image, but assures that the image will have uniform height and width. If you use this approach, then kindly open wp-content\themes\flashlight\woocommerce-config\config.php and find this line of code:
$avia_config['imgSize']['shop_catalog'] = array('width'=>200, 'height'=>140);
and replace it with:
$avia_config['imgSize']['shop_catalog'] = array('width'=>200, 'height'=>140, 'crop'=>true);
and use the Regenerate plugin: http://wordpress.org/extend/plugins/regenerate-thumbnails/
to regenerate all images.