Documentation:
Image Box

This toggle is only visible when the Link setting is set to Link to URL or Link to Lightbox.

Image Overlay

If enabled, an overlay color and icon will be displayed when a user hovers over the image.

Hover Overlay Color

Here you can define a custom color for the overlay.

Overlay Icon Color

Here you can define a custom color for the overlay icon.

Hover Icon Picker

Here you can define a custom icon for the overlay.

See here for more details.