I wanted to follow up with a fix that others might find helpful. The trans.gif image was inserted in a page. Google Webmasters will show you the sitemap where the image exists, but it could be 1000 urls. I'm not sure, but I'm guessing you can configure Yoast to a smaller grouping of URL's. What I did was look at each URL that had 2 or more images and I would drop the link into the demo version of Screaming Spider, with the image URL window open. Most of these will come back as wp-content/... I went through about 60 or so URLs and then bingo. The culprit was a more tag with the trans.gif image at the end. Edited that page and removed the image.
Hope this helps.