Issue when displaying ver & hor pics

Hi there,
I have some problems with the plugin.
When I display several images, some with a horizontal format and some other vertical, the horizontal ones are too much bigger and don’t take the height of the vertical pictures (dunno if it’s understandable…).

Here’s a link to better understand : http://www.maxence-beranger.com/chirurgie-chimique/

Thanks in advance !

http://wordpress.org/plugins/bxslider-wp/