Relative URL to absolute URL Scrapy

I need help to convert relative URL to absolute URL in Scrapy spider.

I need to convert links on my start pages to absolute URL to get the images of the scrawled items, which are on the start pages. I unsuccessfully tried different ways to achieve this and I'm stuck. Any suggestion?

class ExampleSpider(scrapy.Spider):
    name = "example"
    allowed_domains = ["example.com"]
    start_urls = [
        "http://www.example.com/billboard",
        "http://www.example.com/billboard?page=1"
    ]

def parse(self, response):
    image_urls = response.xpath('//div[@class="content"]/section[2]/div[2]/div/div/div/a/article/img/@src').extract()
    relative_url = response.xpath(u'''//div[contains(concat(" ", normalize-space(@class), " "), " content ")]/a/@href''').extract()

    for image_url, url in zip(image_urls, absolute_urls):
        item = ExampleItem()
        item['image_urls'] = image_urls

    request = Request(url, callback=self.parse_dir_contents)
    request.meta['item'] = item
    yield request