Samples and thumbnails for new posts will have a white background.
Samples and thumbnails for old posts (uploaded in Danbooru 1) will likely have black backgrounds, although other solid colors such as hot pink, lime green or grey occasionally occur.
If the non-transparent part of the image is entirely in that background color, the result is a solid rectangle that gives no indication of the original image's content.
Make sure you're viewing the original image, as resized sample images are JPEG and will not have a transparent background.
