A thumbnail picture is kind of like your second book cover. Basically, they're pictures that would represent your novel. When starting up a novel, you need at least a thumbnail and a book cover, and I think the others are optional. You also need thumbnails for your novel's chapters/episodes. However, you don't just upload a picture that easily, your pictures need certain sizes and dimensions for uploading. You can make your own thumbnails and covers in Canva, too! I was also confused while making them, but I've gotten the hang of it! My novel is also quite new, sooo...
I'm sorry if I wasn't able to clarify everything up, I'm not an admin or a Tapas staff member, so if I've said something that's not quite right, feel free to correct me!