If you want an even spacing between images, you will need to use both gutter and marginbottom, setting to equal heights. Note that gutter expects a number, while marginbottom expects a CSS value. That means, for equal spacing, you would need to enter values like this:
15 pixels between
[featured_image_pro gutter=15 marginbottom=15px]
25 pixels between
[featured_image_pro gutter=25 marginbottom=25px]0 pixels between
[featured_image_pro gutter=0 marginbottom=0]If you’re looking for more vertical space, simply add to marginbottom in relation to gutter.
More vertical space
[featured_image_pro gutter=5 marginbottom=25px]