Fixed Type of the gallery has it’s unique properties. You can set the width and height of the gallery in the Fixed type. But if the picture is smaller, it will be cropped accordingly to the set size.
Also, all the pics in this type of gallery will be transformed to the sizes, you have established here. So to get perfect gallery, you have to take all pics with the same sizes.
You can choose it in Gallery Type section of Main Settings Tab.
For example —