How to make lightbox media thumbnails square

Hello everybody!

I’m searching few hours already and can’t find the right answer on the forum. I have a problem with the image sizes in lightbox thumbnails when I open the gallery. It seems that for many people it covers the thumbnails automatically, into squares, but for me they all have different sizes.

To summarize:

How to make this

into this:

I will be really grateful for any help, thank you!

Here is my site Read-Only: