Through the Insert Gallery Window and through the dashboard manage gallery area, 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 default crop for the image is at someones feet from the picture, you can edit the thumbnail preview to be centered at the face. Follow the steps below to learn how to create new thumbnails, and customize the thumbnail preview:
CREATE NEW THUMBNAILS
Through the Insert Gallery Window or dashboard under Gallery >> Manage Gallery, select the gallery you wish to create new thumbnails. Then select all images and within the bulk actions drop-down select 'create new thumbnails'
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!
CUSTOMIZE THUMBNAIL PREVIEW CROP
Go to Manage Gallery > select your gallery you wish to customize the thumbnail preview for an image. Hover over the thumbnail and a list of links should appear, click 'Edit Thumb':
A pop up window will appear, where you can drag your mouse over the preview image to the left to create a new crop for the thumbnail:
Click 'Update' and wait for the confirmation note next to the button, then exit out of that pop up window and click 'Save Changes', and you're done!