Img size

Hallo. Need help. I use several oQey Gallerys in my web site. All images in every gallery is of different size. some of them fit perfect within its proportions but some of them are automatically cut (even as a square) and i have no idea how to fix it. I tried to change it in css, no use. Tried to change the size of gallery in oQey Gallery settings – images change size as well and don’t fit.

https://wordpress.org/plugins/oqey-gallery/