CSF_Field_image_select can have a simple change, allowing a title for the so when the user hovers it, some text shows.

echo '<img src="'. esc_url( $option ) .'" alt="img-'. esc_attr( $num++ ) .'" />';

replace with

echo '<img title="' . esc_attr( $key ) . '" src="'. esc_url( $option ) .'" alt="img-'. esc_attr( $num++ ) .'" />';