Documentation:
Image Box

Here you control how the image looks in the image box. There are separate settings for desktop, tablet, and phone.

Size By

Here you can choose between a preset or custom size for the image within the image box.

Size

Here you can choose a preset size for the image within the image box. Options include:

1. Resize image to fit its box
2. Resize image to fill its box
3. Squish image to fill its box

See here for more details.

See here for an example.