thumbnail line break css

You can achieve this by using the .ngg-gallery-thumbnail CSS class. An example would be:

.ngg-gallery-thumbnail {
    height: 200px; //The height will always be 200px
}

or

.ngg-gallery-thumbnail {
    min-height: 200px; //The height will be at least 200px
}

Do note that this requires you to set the height in the CSS as large as your largest thumbnail, e.g. if your largest thumbnail is 600px, your CSS needs 600px or higher.
That’s why I actually recommend using uniform thumbnail sizes, but if you don’t forget the above, you can use different sizes without problem.