Suwako has a different transparency from the rest of the background, so if you view the image against a background color that isn't white, you can see her. Try clicking the "Size:" link in the Information section on the left, or right-clicking Sanae and selecting "view image."
By copying the following into the URL of a bookmark (after clearing it, of course), clicking it will turn transparencies on Danbooru bright red. Good for both detecting it to tag and for revealing silly things now that our thumbnailer doesn't turn it black any more.
