Wrong size?!

Hello,

I wanted to use NextGen gallery in my new wordpress

Unfortunately the size doesn’t seem to work in a slideshow and a thumbnail strip

as you can see here: https://www.stylemevegan.com

The thumbnail at the slideshow and thumbnail strip is always the same, but shouldnt be
When I use the normal mediathek of wordpress it works and has the correct size

The theme creater told me nextgen gallery is supposed to work, because they used it for several customers :/

Anyone can help?

Thanks!