Avatar thumbnails are not displayed anywhere in the big avatar details menu, yet are shown in the small avatar details menu.
This is since avatars could have gallery images, but whether they do or don't have gallery images (which most don't as it's a new feature) it doesn't display the thumbnail image.
This is especially bad for older avatars that have no gallery images, where the thumbnail clearly displays or describes the avatar, and a lot of creators spend time making custom thumbnails only to not be able to properly view them in the menu.