U-Member image size

I am using University Theme and the U-Member plugin.
I created three members, each with different image. Each image is displayed differently while listing the members on the page. The image sizes are as follows:
(1) 1512 x 2016 px
(2) 427 x 569 px
(3) 427 x 570 px
The image (1) is displayed as cut to square. Images (2) and (3) are displayer as rectangles, but with different height.
What is causing that?
Is it possible to set format of the thumbnails to 4:3?

You can find the result at page
on tab “Profesorowie i Doktorzy Habilitowani”