When uploading your product images, it is recommended to simply upload the largest images you have (while also running them through an image compression tool). Shopify will resize this image to various smaller sizes that will be used throughout your shop. Both landscape and portrait images are supported in all of our themes.
While we recommend uploading product images that are all the same size and orientation, if your product images have different dimensions, then they may appear misaligned on the Collection page.
In order to align them better, you can head into the Theme Editor (customize) > Theme Settings > Product Grid and enable the "align height of product images" setting and adjust the "Product image height" slider. This will set the height of all of your images so they appear aligned in each row.