자답입니다.

<query id="deleteImageList" action="delete">
    <tables>
        <table name="wedding_gallery_images" alias="img" />
        <table name="files" alias="fil" type="left join">
            <conditions>
                <condition operation="equal" column="img.file_srl" default="fil.file_srl" filter="number" notnull="notnull" />
            </conditions>
        </table>
    </tables>
    <conditions>
        <condition operation="equal" column="fil.upload_target_srl" var="document_srl" filter="number" />
    </conditions>
</query>

이렇게 해서 debugPrint 찍어보니 아래처럼 나오네요.

Object Object
(
    [error] => 0
    [message] => success
    [variables] => Array
        (
            [_query] => DELETE `img` FROM `xe_wedding_gallery_images` as `img` left join `xe_files` as `fil` on `img`.`file_srl` = `fil`.`file_srl` WHERE `fil`.`upload_target_srl` = 26051
            [_elapsed_time] => 0.00131
        )

    [httpStatusCode] => 
)

잘 지워지네요.