Considerations for crawling a browser header in a Dynamic Web page

In the original crawl Dynamic Web page image, get the actual address of the picture, but the downloaded image is corrupted, the reason is that the server side blocked the access, but observed that

headers = {'user-agent': Random.choice (useragent_list),           '  Accept'"text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8  ",           'accept-encoding'gzip  ',           }

The browser header is already constructed. What is the reason?

In fact, the dynamic page is from the original page to the image server, so in the browser header need to add the original address, forged out of the browser to jump from the effect.

Crawling Dynamic Web pages Considerations for constructing a browser header