SystemShock added a post in a topic: Crop Thumbnails?
You've posted in the wrong forum which is probably why you haven't received an answer. You're supposed to ask for support here.
I'm not sure if this is what you're trying to achieve but you can try the following.
Log in to your AdminCP and go to Components -> IP.Gallery -> Settings
About half way down the page you will find Image Settings: Thumbnails. You can set your thumbnail size here.
Now go to IP.Gallery -> Tools
The first tool on the right under Image Related Tools is Rebuild Images. Clicking it will rebuild all your images including thumbnails.