This fall, Yahoo began serious talks to buy photo-sharing site Imgur, a source with first-hand knowledge of those discussions tells us. Since she joined Yahoo in July 2012, CEO Marissa Mayer has acquired dozens of startups. Most of these acquisitions have been acqui-hires. The buy that cost Yahoo the most was its $1.1 billion purchase of Tumblr. Yahoo bought Tumblr because it has a deeply engaged, youthful audience, that uses the product on mobile. It would buy Imgur for all the same reasons.