Forum volunteers are not given access to commercial products, so they would not know why it is not working properly. Other community members who may have faced your issue might be able to help you but your best bet is your product’s developer. Keep in mind we encourage you to use the official support venues, as it allows the developers to be aware of issues with their code and gives back to the community in a more robust way.

.page-id-321 .jetpack-portfolio-shortcode.column-4 {
    display: flex;
    flex-wrap: wrap;
    justify-content: space-evenly;
}
.page-id-321 .portfolio-entry-column-4 {
   margin-right: 0;
}

So this should horizontally center the images. It will still look like there’s a big black border at the bottom because you can’t see the text that’s associated with the image. That is, there’s a rule that looks like this:

.page-id-321 .jetpack-portfolio-shortcode.column-4 {
    background: #000;
    text-align: right;
    width: 100% !important;
}

And the black background is hiding the caption of each image, so it looks like there’s a lot of empty space at the bottom. If you change the background to white, you’ll see the text and it won’t look like it’s vertically off-centered.

If you do want to hide the text, there are better ways of doing it instead of putting in a black background.