With the galleries, you may choose the way your images fill their containers.
Cover Vs. Contain
Image Fitting: Cover
Cover tells the browser to make sure the image always covers the entire container, even if it has to stretch the image or cut a little bit off one of the edges.
Image Fitting: Contain
Contain, on the other hand, says to always show the whole image, even if that leaves a little space to the sides or bottom.