You can use this css to add background image in call to action and announcement widgets areas.
//background image in call to action area
background: url( ' Place your Image URL here ' );
//background image in announcement widget areas
background: url(' Place your Image URL here ' );
To retrieve image URL please follow following steps:
1> Go to dashboard-> Media :
2> Click on image you want to show in widgets areas.
To the right of image you will find URL field and copy that image url and placed it inside your Url field in above css.
Note: Please use child theme to add custom css, otherwise your changes will be lost on theme update.