Let's assume you added a new image size of 680px by 400px and re-processed your images. Now, open up the index.php file in the gallery directory you are using. On about line 14, there should be some code like this: &thumb=62c&full=782p. Change the 782p to 680p. That will use your new image.
Basically, the 2 arguments for the thumb and full can be changed to the size you need. The 782 is the width, p is for proportional image, c is for cropped. So by default it uses the 782px wide image that is proportionally sized.
Updates like this are more complex so that is why asking in the forum is a good place for now.