Documentation:
Image Box

Images can include a caption which can be loaded dynamically from the media attachment or entered manually.

Caption

Here you can select the source of the caption text. There are six options:

1. None
2. Title Text
3. Alternative Text
4. Caption
5. Description
6. Custom

When Title Text or Alternative Text is selected, values from the Attributes toggle will be used first. If no values are found there, values will be taken from the media attachment. When Caption or Description is selected, values will always be taken from the media attachment.

See here for more details.

Custom Caption

Here you can enter a custom caption.

See here for positioning the caption.

See here for styling the caption.

See here for an example.