Tumblr has recently introduced Sponsored Posts. These posts can come from anyone, regardless of whether or not you follow them, and are primarily for advertising. If you want to block them, simply install Tumblr Savior and blacklist “sponsored_badge_icon” (without the quotes).
Oh wow OK let’s please signal boost this because some recent ones have been GIFs of horror and so on, and that makes Tumblr incredibly unsafe for a good amount of people. S: