Some thumbnails rectangle and some square

Hello,

I’ve setted YARPP to display 4 related post with thumbnails, I’ve customized them to be 160px width and heigth is auto.
The two firsts thumbnail are displayed well, but the two others are stretched to fit a square :
http://www.photographe-mariage-grenoble.pro/mariage-pyu-en-velay/

Any ideas what is the problem?

Thank you

http://ww.wp.xz.cn/extend/plugins/yet-another-related-posts-plugin/