You can change your gallery thumbnail size on a per gallery basis. You can also customize the thumbnail preview for each thumbnail. For example, if the thumbnail photo is off center or cutting part of the image off, you can edit the thumbnail preview to be centered on what you want to be most important.
Create New Thumbnails
Select Manage Gallery by clicking the NextGEN icon in the toolbar of your page or post, (or from the dashboard – Gallery – Manage Galleries). Select the gallery you wish to create new thumbnails. Then select Create new thumbnails from the bulk actions dropdown.
The process may take a few minutes or several minutes depending on how large your gallery is. Once the process is complete, be sure to click the Save Changes button!
Once the thumbnail is created you can simply edit the crop of the thumbnail to have the image appear the way you want by clicking the Edit Thumb underneath the image.
You can crop the thumbnail image to the size and part of the photo you want. Click Update.
Be sure to click Save Changes.