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.