Through the Insert Gallery Window ('Add Gallery' button in post/page edit) you can change your gallery thumbnail size on a per gallery basis under Insert Into Page > Customize Display Settings for Basic Thumbnail, Pro Thumbnail, and Pro Film gallery display's. You may also modify the thumbnail size for Basic Compact, Basic Extended, Pro Grid, and Pro List Album displays.
Thumbnail Preview Image
You can also customize the thumbnail preview image for each thumbnail under the 'Add Gallery' window > Manage Galleries, or under Gallery > Manage Galleries in the dashboard. For example, if the default crop for the image is at the center of a lavendar field, you can edit the thumbnail preview to be centered at the tree in the lavendar field. Follow the steps below to learn how to customize the thumbnail preview, and create new thumbnails:
CUSTOMIZE THUMBNAIL PREVIEW CROP
Through the 'Add Gallery' window or dashboard under Gallery > Manage Gallery, select the gallery to edit. Under the thumbnail is a list of links, click 'Edit Thumb':
A pop up window will appear, where you can drag your mouse over the preview image to create a new crop for the thumbnail:
Click 'Update' and wait for the confirmation note under the button, then exit out of that pop up window and click 'Save Changes', and you're done!
CREATE NEW THUMBNAILS
Through the 'Add Gallery' window or dashboard under Gallery > Manage Gallery, select the gallery to edit. In your list of images select the box next to the image thumbnail and under the bulk actions drop-down select 'create new thumbnails', set the size of the thumbnail you want to create, and then click the 'Apply' button to start creating 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!