Please follow the next steps in order to setup resizing and cropping correctly:
- Login to your WordPress admin dashboard as administrator.
- Go to the Gallery plugin settings page.
- Set necessary size for the Gallery image size and Crop position.
- Re-upload your images.
Click "Update Images" button on the plugin settings page, and wait for all images to get updated.
WordPress will create a copy of the thumbnail with the specified dimensions when you upload a new image.