With the Web page in question loaded into your browser, view the HTML source code of the page.
Find the <img> tag for the offending advert. This can be quite awkward to do. You might like to try the following.
Point to the advert in the browser window, and note a unique part of the URL that it links to from the status bar. Search for this in the source code window.
Find the alt text of the image. This often appears as the image loads. Try turning off picture loading - some browsers will show you the alt text instead. Search the source for some of the text in the alt tag.
In the <img> tag, examine the src item. Note down its URL. This might be a URL relative to the page you are on - if so note down the full URL. You can try going to it to check it is right - you should see just the advert in your browser window.
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