How to adjust the image thumbnail in the Gallery

Can you advise how to adjust the picture thumbnail on the gallery page?
Pictures are showing with the heads cut off. See website below.

http://www.sidneygracephoto.com/gallery/

 

Hello,

You are adding portrait images of long height. That's why images are cropping.
There are two solutions through which you can control this cropping.
1) Try to add landscape images.
2) If you only want to add portrait.Then add small images and also you can increase cropping size through "template-gallery.php"
Go to Appearance > Editor > template-gallery.php
Check image for reference.

 

This worked... thanks so much!